The CircleCI x86_64/Linux LLVM way exhibits numerous testsuite failures:

Unexpected results from:
TEST="CPUTime001 ClosedFam1TH T10828 T10891 T11341 T11345 T11463 T11721_TH T11797 T12403 T12646 T12962 T13642 T13887 T14060 T1835 T2222 T2552 T2700 T3920 T4135 T4188 T5037 T5358 T5362 T5363 T5559 T680 T7477 T8761 T8884 T8953 T9064 T9262 T9692 TH_PromotedList TH_RichKinds TH_RichKinds2 TH_Roles3 TH_TyInstWhere2 TH_foreignCallingConventions TH_reifyDecl1 TH_reifyDecl2 TH_reifyInstances TH_repE2 TH_repGuard TH_repPrim TH_repPrim2 TH_repUnboxedTuples posix002 prof-doc-fib prof-doc-last profinline001 scc001 scc002 scc003 scc005"

SUMMARY for test run started at Mon Jun 18 08:57:25 2018 UTC
 1:18:58 spent to go through
    6443 total tests, which gave rise to
   25148 test cases, of which
    4810 were skipped

     229 had missing libraries
   19825 expected passes
     227 expected failures

       0 caused framework failures
       0 caused framework warnings
       0 unexpected passes
      57 unexpected failures
       0 unexpected stat failures

Unexpected failures:
   profiling/should_run/scc001.run                   scc001 [bad exit code] (ghci-ext-prof)
   profiling/should_run/scc002.run                   scc002 [bad exit code] (ghci-ext-prof)
   profiling/should_run/scc003.run                   scc003 [bad exit code] (ghci-ext-prof)
   profiling/should_run/scc005.run                   scc005 [bad exit code] (ghci-ext-prof)
   profiling/should_run/T680.run                     T680 [bad exit code] (ghci-ext-prof)
   profiling/should_run/T2552.run                    T2552 [bad exit code] (ghci-ext-prof)
   profiling/should_run/prof-doc-fib.run             prof-doc-fib [bad exit code] (ghci-ext-prof)
   profiling/should_run/T5559.run                    T5559 [bad exit code] (ghci-ext-prof)
   profiling/should_run/prof-doc-last.run            prof-doc-last [bad exit code] (ghci-ext-prof)
   profiling/should_run/profinline001.run            profinline001 [bad exit code] (ghci-ext-prof)
   profiling/should_run/T5363.run                    T5363 [bad exit code] (ghci-ext-prof)
   profiling/should_run/T12962.run                   T12962 [bad exit code] (ghci-ext-prof)
   th/TH_repPrim.run                                 TH_repPrim [exit code non-0] (ext-interp)
   th/TH_repPrim2.run                                TH_repPrim2 [exit code non-0] (ext-interp)
   th/TH_repUnboxedTuples.run                        TH_repUnboxedTuples [exit code non-0] (ext-interp)
   th/TH_repGuard.run                                TH_repGuard [exit code non-0] (ext-interp)
   th/TH_repE2.run                                   TH_repE2 [exit code non-0] (ext-interp)
   th/TH_reifyDecl1.run                              TH_reifyDecl1 [exit code non-0] (ext-interp)
   th/TH_reifyDecl2.run                              TH_reifyDecl2 [exit code non-0] (ext-interp)
   th/TH_reifyInstances.run                          TH_reifyInstances [exit code non-0] (ext-interp)
   th/T2700.run                                      T2700 [exit code non-0] (ext-interp)
   th/TH_foreignCallingConventions.run               TH_foreignCallingConventions [exit code non-0] (ext-interp)
   th/T4188.run                                      T4188 [exit code non-0] (ext-interp)
   th/T3920.run                                      T3920 [exit code non-0] (ext-interp)
   th/T5037.run                                      T5037 [exit code non-0] (ext-interp)
   th/T5362.run                                      T5362 [exit code non-0] (ext-interp)
   th/T1835.run                                      T1835 [exit code non-0] (ext-interp)
   th/T5358.run                                      T5358 [stderr mismatch] (ext-interp)
   th/TH_PromotedList.run                            TH_PromotedList [exit code non-0] (ext-interp)
   th/TH_RichKinds.run                               TH_RichKinds [exit code non-0] (ext-interp)
   th/TH_RichKinds2.run                              TH_RichKinds2 [exit code non-0] (ext-interp)
   th/T4135.run                                      T4135 [exit code non-0] (ext-interp)
   th/TH_TyInstWhere2.run                            TH_TyInstWhere2 [exit code non-0] (ext-interp)
   th/T2222.run                                      T2222 [exit code non-0] (ext-interp)
   th/ClosedFam1TH.run                               ClosedFam1TH [exit code non-0] (ext-interp)
   th/TH_Roles3.run                                  TH_Roles3 [exit code non-0] (ext-interp)
   th/T7477.run                                      T7477 [exit code non-0] (ext-interp)
   th/T8884.run                                      T8884 [exit code non-0] (ext-interp)
   th/T9262.run                                      T9262 [exit code non-0] (ext-interp)
   th/T9692.run                                      T9692 [exit code non-0] (ext-interp)
   th/T8953.run                                      T8953 [exit code non-0] (ext-interp)
   th/T9064.run                                      T9064 [exit code non-0] (ext-interp)
   th/T10828.run                                     T10828 [exit code non-0] (ext-interp)
   th/T10891.run                                     T10891 [exit code non-0] (ext-interp)
   th/T11341.run                                     T11341 [exit code non-0] (ext-interp)
   th/T11345.run                                     T11345 [exit code non-0] (ext-interp)
   th/T11721_TH.run                                  T11721_TH [exit code non-0] (ext-interp)
   th/T11797.run                                     T11797 [exit code non-0] (ext-interp)
   th/T11463.run                                     T11463 [exit code non-0] (ext-interp)
   th/T8761.run                                      T8761 [exit code non-0] (ext-interp)
   th/T12403.run                                     T12403 [exit code non-0] (ext-interp)
   th/T12646.run                                     T12646 [exit code non-0] (ext-interp)
   th/T13642.run                                     T13642 [exit code non-0] (ext-interp)
   th/T13887.run                                     T13887 [exit code non-0] (ext-interp)
   th/T14060.run                                     T14060 [exit code non-0] (ext-interp)
   ../../libraries/base/tests/CPUTime001.run         CPUTime001 [bad stdout] (threaded2)
   ../../libraries/unix/tests/libposix/posix002.run  posix002 [bad exit code] (threaded2)

Unfortunately, most of these appear to be segmentation faults and similar, suggesting miscompilation.