c-family: Fix ICE in get_atomic_generic_size [PR96545]

As the testcase shows, we would ICE if the type of the first argument of
various atomic builtins was pointer to (non-void) incomplete type, we would
assume that TYPE_SIZE_UNIT must be non-NULL.  This patch diagnoses it
instead.  And also changes the TREE_CODE != INTEGER_CST check to
!tree_fits_uhwi_p, as we use tree_to_uhwi after this and at least in theory
the int could be too large and not fit.

2020-08-11  Jakub Jelinek  <jakub@redhat.com>

	PR c/96545
	* c-common.c (get_atomic_generic_size): Require that first argument's
	type points to a complete type and use tree_fits_uhwi_p instead of
	just INTEGER_CST TREE_CODE check for the TYPE_SIZE_UNIT.

	* c-c++-common/pr96545.c: New test.