[clang] [clang] Expose MC target options through -mllvm (PR #210087)

[Farid Zakaria via cfe-commits]

fzakaria wrote:

CC @efriedma-quic @AaronBallman @asl  -- I'm not sure who the Codegen maintainer is; I tried my best to pick those listed in the [maintainers](https://github.com/llvm/llvm-project/blob/main/clang/Maintainers.md) file.

:pray: please recommend appropriate reviewer if you know it.

Thanks.

https://github.com/llvm/llvm-project/pull/210087