c: Diagnose auto constexpr used with a type

The constraints on auto in C2x disallow use with other storage-class
specifiers unless the type is inferred from an initializer.  That
includes constexpr; add the missing checks for this case (the
combination of auto, constexpr and a type specifier).

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

gcc/c/
	* c-decl.cc (declspecs_add_type, declspecs_add_scspec): Check for
	auto, constexpr and a type used together.

gcc/testsuite/
	* gcc.dg/c2x-constexpr-1.c: Do not use auto, constexpr and a type
	together.
	* gcc.dg/c2x-constexpr-3.c: Add tests of auto, constexpr and type
	used together.