[clang] [clang] Fix constant evaluation of member pointer access into sibling class. (PR #150829)

[Justin Stitt via cfe-commits]

JustinStitt wrote:

> @erichkeane Thank you.
> 
> I realized that both issues fixed by this PR are still present in similar form if `-fexperimental-new-constant-interpreter` is used 

cc @tbaederr 


https://github.com/llvm/llvm-project/pull/150829