c: Update checks on constexpr pointer initializers

c: Update checks on constexpr pointer initializers

WG14 has agreed a change of the rules on constexpr pointer
initializers, so that a (constant) null value that is not a null
pointer constant is accepted in that context, rather than only
accepting null pointer constants.  (In particular, this means that a
constexpr variable of pointer type can be used to initializer another
such variable.)  Remove the null pointer constant restriction in GCC,
instead checking just whether the value is null.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/c/
	* c-typeck.cc (check_constexpr_init): Remove argument
	null_pointer_constant.  Only check pointer initializers for being
	null.
	(digest_init): Update calls to check_constexpr_init.

gcc/testsuite/
	* gcc.dg/c2x-constexpr-1.c: Test initialization of constexpr
	pointers with null values that are not null pointer constants.
	* gcc.dg/c2x-constexpr-3.c: Test initialization of constexpr
	pointers with non-null values, not with null values that are not
	null pointer constants.