[clang] [RISCV] Add -march string as Module metadata in IR. (PR #80760)

[Alex Bradbury via cfe-commits]

asb wrote:

I agree that finding a generic cross-target solution to this problem is attractive...but I worry it will be hard and slow to proceed. It's possible the previous discussions just lacked someone following up with concrete patches, but I do worry we'd end up stuck in limbo vs starting with a target-specific approach that we later hope to replace with something more generic.

With that in mind:
* To what degree are we constrained by backwards compatibility once we commit to something like this? It's hard to imagine not being able to write an appropriate IR autoupgrade rule, but perhaps I'm missing something.
* Just to check my understanding, is the only usecase for this ISA naming string to produce appropriate ELF attributes?

Two simple and early pieces of feedback on the patch:
* Nitpick: We're not adding the -march string as module metadata, we're adding an ISA naming string in a normalized form.
* A comment in CodeGenModule or at least the test to briefly indicate _why_ this metadata is wanted would be helpful I think.

https://github.com/llvm/llvm-project/pull/80760