c: fix checking ICE related to transparent unions and atomic [PR123309]

c: fix checking ICE related to transparent unions and atomic [PR123309]

When matching function arguments in composite_type_internal and one
type comes from a transparent union, it is possible to end up with
atomic and non-atomic types because this case is not handled correctly.
The type matching logic is rewritten in a cleaner way to use helper
functions and to not walk the argument lists three times.  With this
change, a checking assertion can be added to test for matching qualifiers
for pointers. (In general, this assumption is still violated for
function return types.)

	PR c/123309

gcc/c/ChangeLog:
	* c-typeck.cc (transparent_union_replacement): New function.
	(composite_type_internal): Rewrite logic.
	(type_lists_compatible_p): Remove dead code for NULL arguments.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr123309.c: New test.
	* gcc.dg/union-composite-type.c: New test.