[clang] [Driver] Normalize the baremetal handling of libc++ and runtimes (PR #101259)

[Petr Hosek via cfe-commits]

petrhosek wrote:

> Gentle ping @petrhosek

I'd like to go ahead and merge this PR but I still need someone to approve this change.

https://github.com/llvm/llvm-project/pull/101259