[Concepts] Fix placeholder constraints when references are involved

Placeholder types were not checked for constraint satisfaction when modified by references or pointers.
The behavior now matches that of GCC and MSVC.

Are there other modifiers we might need to "peel"? I'm not sure my approach to this is the 'right' way to fix this, the loop feels a bit clunky.

GitHub issues [[ https://github.com/llvm/llvm-project/issues/54443 | #54443 ]], [[ https://github.com/llvm/llvm-project/issues/53911 | #53911 ]]

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D122083