c: Error on va_arg with function type [PR105149]

c: Error on va_arg with function type [PR105149]

In the PR Joseph said that the C standard for va_arg talks about
pointers to object type and as a function type is not object type,
it is invalid.

The following patch diagnoses it in the FE, instead of ICEing later on
when optimizations are turned on (and with -O0 doing something weird
at runtime).

2022-04-08  Jakub Jelinek  <jakub@redhat.com>

	PR c/105149
	* c-typeck.cc (c_build_va_arg): Reject function types.

	* gcc.dg/pr105149.c: New test.