c: Fix construction of composite type for atomic pointers [PR121081]

c: Fix construction of composite type for atomic pointers [PR121081]

When constructing the composite type of two atomic pointer types,
we used "qualify_type" which did not copy the "atomic" qualifier.
Use c_build_type_attribute_qual_variant instead.

	PR c/121081

gcc/c/ChangeLog:
	* c-typeck.cc (composite_type_internal): Properly copy
	atomic qualifier.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr121081.c: New test.