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

[Victor Campos via cfe-commits]

vhscampos wrote:

Thanks @smithp35. Before any of the PRs is merged, I will update the descriptions to make them more self-contained.

Furthermore, `Multilib.rst` must indeed be updated. Will do it.

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