c-family: Avoid ICEs on calls to internal functions [PR95963]

c-family: Avoid ICEs on calls to internal functions [PR95963]

The following testcase ICEs since recent Martin's -Wnonnull changes,
we see a CALL_EXPR and ICE because CALL_EXPR_FN is NULL, which is
valid for internal function calls.  Internal function calls don't have a
function type, and will never have format_arg attribute on them nor will
serve as the i18n routines -Wformat cares about.

2020-06-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/95963
	* c-common.c (check_function_arguments_recurse): Don't crash on
	calls to internal functions.

	* g++.dg/cpp1z/launder9.C: New test.