[clang] [llvm] [LLVM][Clang][AArch64] Implement AArch64 build attributes (PR #118771)

[Oliver Stannard via llvm-commits]

ostannard wrote:

The code changes are all in llvm (not clang), so the tests need to be there too, using `llvm-mc` and `llc` instead of `clang`.

I'd prefer to see the code for the assembly parser before reviewing this, because that will probably affect the API design of the streamer, since it will need to handle switching back and forth between subsections in ways llc doesn't need.

https://github.com/llvm/llvm-project/pull/118771