[clang] Correctly(?) handle placeholder types in ExprRequirements.

Bug #52905 was originally papered over in a different way, but
I believe this is the actually proper fix, or at least closer to
it. We need to detect placeholder types as close to the front-end
as possible, and cause them to fail constraints, rather than letting
them persist into later stages.

Fixes #52905.
Fixes #52909.
Fixes #53075.

Differential Revision: https://reviews.llvm.org/D118552