[Clang] Modify CXXMethodDecl::isMoveAssignmentOperator() to look through type sugar

AcceptedPublic

Currently CXXMethodDecl::isMoveAssignmentOperator() does not look though type
sugar and so if the parameter is a type alias it will not be able to detect
that the method is a move assignment operator. This PR fixes that and adds a set
of tests that covers that we correctly detect special member functions when
defaulting or deleting them.

This fixes: https://github.com/llvm/llvm-project/issues/56456

Differential Revision: https://reviews.llvm.org/D129591