[clang] [RISCV] Merging RISCVToolChain and BareMetal toolchains (PR #118809)

[Kito Cheng via cfe-commits]

kito-cheng wrote:

I would suggest to break this PR into several small pieces, the clang/test folder should not having too much change during the merging, especially I feel not conformable changing the non-RISC-V file within this PR, I expect those change should happened in a separated patch.

https://github.com/llvm/llvm-project/pull/118809