[clang] [libc] [clang] diagnose invalid member pointer class on instantiation (PR #132516)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

> Why was this landed w/o any approvals?

I am reading the PR and I see now this was a fix for something you just landed but that was not clear from the summary in the PR. 

https://github.com/llvm/llvm-project/pull/132516