[Clang] Update warning on some designator initializer cases involving unions

Currently when using designated initializers in C++ we have a few
extension. Two extension which are dangerous involved assigning to
multiple members of union which will likely be a mistake since unions
can only have one active member. I have updated to be a warning by
default.

The second case if when we assign to multiple union members and one of
the previous members had a non-trivial destructor, which could lead to
leaking resources. This one is now an error by default.

Fixes: https://github.com/llvm/llvm-project/issues/62156

Differential Revision:  https://reviews.llvm.org/D149694