c: Fix ICE when using va_arg with arrays [PR97986]

c: Fix ICE when using va_arg with arrays [PR97986]

When array type is passed to va_arg, this is undefined behavior.
Emit a warning, and insert a run-time trap after evaluating side effects,
but return the correct type for sizeof / typeof.  For C90 a VLA is an
error.

	PR c/97986

gcc/c/ChangeLog:
	* c-parser.cc (c_parser_postfix_expression): Adapt.
	* c-tree.h (c_build_va_arg): Adapt prototype.
	* c-typeck.cc (c_build_va_arg): Handle UB.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr97986-1.c: New test.
	* gcc.dg/pr97986-2.c: New test.