[clang] [Multilib] Extend the Multilib system to support an IncludeDirs field. (PR #146651)

[Petr Hosek via cfe-commits]

petrhosek wrote:

The implementation looks good modulo a few nits, but after doing some more research on multilibs I'm still not sure if this is the right solution (or rather whether we're solving the right problem to begin with), but I think it'd be better to continue that discussion on the RFC so I'll post my thoughts there.

https://github.com/llvm/llvm-project/pull/146651