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

[Fangrui Song via cfe-commits]

MaskRay wrote:

I share a similar concern about not addressing the issue in a target-specific manner. However, I know a lot of confused users by the behavior, and this patch will significantly improve the status quo. I concur with asb's analysis.

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