[clang] [Clang] Take libstdc++ into account during GCC detection (PR #145056)

[Frederik Harwath via cfe-commits]

frederik-h wrote:

> Signalling my support for the changes. It looks like @MaskRay is also happy with them. So I'd say let's give folks one more week for additional before landing the changes.

Thanks @AaronBallman! I am going to merge the PR tomorrow. 

https://github.com/llvm/llvm-project/pull/145056