[clang] [llvm] [Driver][RISC-V] Support YAML multilib config with selectRISCVMultilib fallback (PR #184973)

[Sam Elliott via llvm-commits]

lenary wrote:

> a user requesting rv32imc won't match an rv32im variant even though it is a safe fallback

I'm not sure this assertion is true, given we are working on a toolchain that uses the multilib yaml and does manage to do this. 

Let me get back to you tomorrow with how we made this work.

https://github.com/llvm/llvm-project/pull/184973