[C++20] [Modules] Use the canonical decl when getting associated constraints

Close https://github.com/llvm/llvm-project/issues/62943.

The root cause for the issue is that we think the associated constraints
from the 'same' declaration in different module units are different
incorrectly. Since the constraints doesn't know anything about decls and
modules, we should fix the problem by getting the associated constraints
from the exactly the same declarations from different modules.