c: Add more C2Y tests

c: Add more C2Y tests

Add further tests for things defined in C2Y that correspond to how GCC
already behaves.  Some of these are for previous undefined behavior
where what is violated has become a constraint but previously was a
"shall" outside Constraints.  Others (c2y-anon-init-1.c and
c2y-incomplete-2.c) reflect semantics that were intended all along but
not correctly or sufficiently clearly stated in the standard.

Tested for x86_64-pc-linux-gnu.

	* gcc.dg/c2y-anon-init-1.c, gcc.dg/c2y-incomplete-2.c:
	gcc.dg/c2y-linkage-1.c, gcc.dg/c2y-linkage-2.c,
	gcc.dg/c2y-linkage-3.c, gcc.dg/c2y-linkage-4.c,
	gcc.dg/c2y-static-assert-1.c: New tests.