Use HDoc for LLVM backend (#22455)

This MR is intended to complete some of the work from #22455 to use the more efficient HDoc type for the LLVM backend. Currently it's lacking:

• almost certainly doesn't meet the style guide
  • SPECIALIZE pragmas should be moved under unction definitions(?)
• Handling metadata is something I'm not happy about - with the current AST, it's possible to recursively have to append an arbitrary amount of metadata to any given statement (which includes comments and labels), which feels wrong. Personally I'd prefer to see the AST modified, perhaps using pattern synonyms, so that only statements that we need to append metadata to contain it. This MR collects up the metadata and achieves the same result.

As an aside, while testing I noticed that master fails several tests when compiling using the LLVM backend, the common aspects seem to be the use of StaticPtr or call stacks:

Unexpected results from:
TEST="CgStaticPointers CgStaticPointersNoFullLazyness GcStaticPointers ListStaticPointers T12622 T18623 T367_letnoescape UnboxedTuples changelogs decodeMyStack decodeMyStack_underflowFrames staticcallstack001 staticcallstack002"

SUMMARY for test run started at Mon Jan 16 23:35:38 2023 
0:33:46.401927 spent to go through
    9232 total tests, which gave rise to
   54652 test cases, of which
   50177 were skipped
       7 had missing libraries

    4402 expected passes
      52 expected failures

       0 caused framework failures
       0 caused framework warnings
       0 unexpected passes
      13 unexpected failures
       0 unexpected stat failures
       8 fragile tests

Unexpected failures:
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/codeGen/should_run/CgStaticPointers.run                CgStaticPointers [bad exit code (1)] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/codeGen/should_run/CgStaticPointersNoFullLazyness.run  CgStaticPointersNoFullLazyness [bad exit code (1)] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/linters/changelogs.run                                 changelogs [bad stdout] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/rts/decodeMyStack.run                                  decodeMyStack [bad stdout] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/rts/decodeMyStack_underflowFrames.run                  decodeMyStack_underflowFrames [bad exit code (1)] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/rts/GcStaticPointers.run                               GcStaticPointers [bad exit code (1)] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/rts/ListStaticPointers.run                             ListStaticPointers [bad exit code (1)] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/profiling/should_run/staticcallstack001.run            staticcallstack001 [bad stdout] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/profiling/should_run/staticcallstack002.run            staticcallstack002 [bad stdout] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/codeGen/should_run/T12622.run                          T12622 [bad exit code (1)] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/rts/T18623/T18623.run                                  T18623 [bad exit code (1)] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/concurrent/should_run/T367_letnoescape.run             T367_letnoescape [bad exit code (99): test timeout] (llvm)
   /var/folders/s7/5wdsz_xn6zxdnzl3hm67b3mc0000gn/T/ghctest-5j0zld9z/test   spaces/testsuite/tests/ghci/should_run/UnboxedTuples/UnboxedTuples.run        UnboxedTuples [bad exit code (138)] (llvm)

Appending 0 stats to file: master-964284fcab6-summary.txt
user error (tests failed)
Build failed.

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