c: Allow bool and enum null pointer constants [PR112556]

c: Allow bool and enum null pointer constants [PR112556]

As reported in bug 112556, GCC wrongly rejects conversion of null
pointer constants with bool or enum type to pointers in
convert_for_assignment (assignment, initialization, argument passing,
return).  Fix the code there to allow BOOLEAN_TYPE and ENUMERAL_TYPE;
it already allowed INTEGER_TYPE and BITINT_TYPE.

This bug (together with -std=gnu23 meaning false has type bool rather
than int) has in turn resulted in people thinking they need to fix
code using false as a null pointer constant for C23 compatibility.
While such a usage is certainly questionable, it has nothing to do
with C23 compatibility and the right place for warnings about such
usage is -Wzero-as-null-pointer-constant.  I think it would be
appropriate to extend -Wzero-as-null-pointer-constant to cover
BOOLEAN_TYPE, ENUMERAL_TYPE and BITINT_TYPE (in all the various
contexts in which that option generates warnings), though this patch
doesn't do anything about that option.

Bootstrapped with no regressions for x86-64-pc-linux-gnu.

	PR c/112556

gcc/c/
	* c-typeck.cc (convert_for_assignment): Allow conversion of
	ENUMERAL_TYPE and BOOLEAN_TYPE null pointer constants to pointers.

gcc/testsuite/
	* gcc.dg/c11-null-pointer-constant-1.c,
	gcc.dg/c23-null-pointer-constant-1.c: New tests.