Deduplicate one-shot/make compile paths.
Summary: We had a duplicate copy of the code for --make and for -c which was a pain. The call graph looked something like this: compileOne -> genericHscCompileGetFrontendResult -> genericHscFrontend hscCompileOneShot ---^ with genericHscCompileGetFrontendResult and hscCompileOneShot duplicating logic for deciding whether or not recompilation was needed. This patchset fixes it, so now everything goes through this call-chain: compileOne (--make entry point) Calls hscIncrementCompile, invokes the pipeline to do codegen and sets up linkables. hscIncrementalCompile (-c entry point) Calls hscIncrementalFrontend, and then simplifying, desugaring, and writing out the interface. hscIncrementalFrontend Performs recompilation avoidance, if recompilation needed, does parses typechecking. I also cleaned up some of the MergeBoot nonsense by introducing a FrontendResult type. NB: this BREAKS #8101 again, because I can't unconditionally desugar due to Haddock barfing on lint, see #10600 Signed-off-by: Edward Z. Yang <email@example.com> Test Plan: validate Reviewers: simonpj, bgamari, simonmar, austin Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1302