[C++20] [Modules] Only allow redeclarations in partitions if they are in the same module

Closes https://github.com/llvm/llvm-project/issues/58196.

The root cause for the problem is an oversight in
https://reviews.llvm.org/D127624, which allows the redeclarations in
partitions. However, we took a mistake there that we should only allow
it if the redeclarations in the one same module instead of return
directly if either the redeclaration lives in a partition. The original
implementation makes no sense and I believe it was an oversight.