c: Implement C2Y alignof on incomplete arrays

c: Implement C2Y alignof on incomplete arrays

C2Y has adopted support for alignof applied to incomplete array types
(N3273).  Add this support to GCC.  As the relevant checks are in
c-family code that doesn't have access to functions such as
pedwarn_c23, this remains a hard error for older versions and isn't
handled by -Wc23-c2y-compat, although preferably it would work like
pedwarn_c23 (pedwarn-if-pedantic for older versions, warning with
-Wc23-c2y-compat in C2Y mode).

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/c-family/
	* c-common.cc (c_sizeof_or_alignof_type): Allow alignof on an
	incomplete array type for C2Y.

gcc/testsuite/
	* gcc.dg/c23-align-10.c, gcc.dg/c2y-align-1.c,
	gcc.dg/c2y-align-2.c: New tests.