Upgrade to LLVM 15, support Apple M1.

[dan-zheng]

Main changes:

• Upgrade to GHC 9.2.x.
  • Apple M1 support seems to be mostly mature starting in GHC 9.2.1: https://www.haskell.org/ghc/blog/20210309-apple-m1-story.html.
• Upgrade to LLVM 15.
  • Use the same resolver in stack*.yaml files as llvm-hs:llvm-15, for consistency.

---

Status: work-in-progress.

• [x] make now works on Apple M1.
  • The easiest path to Apple M1 support I found involved upgrading both GHC (details) and LLVM.
• [x] make unit-tests passes.
• [x] Fix compiler warnings (full log).
  • There are many -Wincomplete-uni-patterns warnings because -Wincomplete-uni-patterns was added to -Wall in GHC 9.2.1 (source).
• [x] make tests passes.
  • Some tests are fixed while others are disabled for now. Test regressions tracked in https://github.com/google-research/dex-lang/issues/1278.

Fixes #1247.

[dan-zheng]

I believe this is ready for review now. @dougalm

• [x] Building with LLVM 15 and GHC 9.2.x works.
• [x] CI passes.
• [x] Test regressions are documented in https://github.com/google-research/dex-lang/issues/1278 and can easily be found via grep -nr "FIXME(llvm-15)" (or rg "FIXME\(llvm-15\)" which is faster).

Let me know if I should do anything else!