[Modules] Don't check [temp.friend]p9 in ASTContext::isSameEntity

[temp.friend]p9 says

> Such a constrained friend function or function template declaration does
not declare the same function or function template as a declaration in
any other scope.

But the friend declaration in the same scope shouldn't fall into this
catagory. Although the logic is handled in 'FriendsDifferByConstraints',
the compiler may haven't merged the lexcial declcontext in time.
Also the code removed is not covered by test too.

Let's handle the logic in sema as we've done now.