[Sema] Merge C++20 concept definitions from different modules in same TU

Currently the C++20 concepts are only merged in `ASTReader`, i.e. when
coming from different TU. This can causes ambiguious reference errors when
trying to access the same concept that should otherwise be merged.

Please see the added test for an example.

Note that we currently use `ASTContext::isSameEntity` to check for ODR
violations. However, it will not check that concept requirements match.
The same issue holds for mering concepts from different TUs, I added a
FIXME and filed a GH issue to track this:
https://github.com/llvm/llvm-project/issues/56310

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D128921