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

[Craig Topper via cfe-commits]

topperc wrote:

> * Just to check my understanding, is the only usecase for this ISA naming string to produce appropriate ELF attributes?

We probably need it to set the `EF_RISCV_RVC` flag too.

I just noticed we only set EF_RISCV_RVC based on C, and not Zca. Is that a bug?

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