[clang] [Multilib] Custom flags YAML parsing (PR #110657)

[Victor Campos via cfe-commits]

vhscampos wrote:

I've reverted this patch because apparently Clang driver does not meld well with `std::shared_ptr`: memory leaks due to custom allocators I think. Thus I've redone the part to use raw pointers instead: https://github.com/llvm/llvm-project/pull/122903

https://github.com/llvm/llvm-project/pull/110657