c++: [[no_unique_address]] and cv-qualified type

c++: [[no_unique_address]] and cv-qualified type

We were checking for overlap using same_type_p and therefore allocating two
Empty subobjects at the same offset because one was cv-qualified.

This gives the warning at the location of the class name rather than the
member declaration, but this should be a rare enough issue that it doesn't
seem worth trying to be more precise.

gcc/ChangeLog:

	* common.opt: Update -fabi-version=19.

gcc/cp/ChangeLog:

	* class.cc (check_subobject_offset): Check
	same_type_ignoring_top_level_qualifiers_p.

gcc/testsuite/ChangeLog:

	* g++.dg/abi/no_unique_address8.C: New test.
	* g++.dg/abi/no_unique_address8a.C: New test.