c: Allow conversions of null pointer constants to nullptr_t

c: Allow conversions of null pointer constants to nullptr_t

WG14 has agreed to allow conversions (explicit and implicit) from null
pointer constants to nullptr_t; update GCC's nullptr_t implementation
to match.

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

gcc/c/
	* c-convert.cc (c_convert): Allow conversion of a null pointer
	constant to nullptr_t.
	* c-typeck.cc (null_pointer_constant_p): Remove static.
	(convert_for_assignment): Allow conversion of a null pointer
	constant to nullptr_t.
	(digest_init): Handle NULLPTR_TYPE among scalar conversions.
	* c-tree.h (null_pointer_constant_p): Declare.

gcc/testsuite/
	* gcc.dg/c2x-nullptr-1.c: Test conversion of null pointer
	constants to nullptr_t.
	* gcc.dg/c2x-nullptr-3.c: Do not expect errors for conversion of
	null pointer constants to nullptr_t.  Do test errors for
	conversion of other values to nullptr_t and for unary '+' on
	nullptr_t.