c++: check DECL_INITIAL for constexpr

c++: check DECL_INITIAL for constexpr

We were overlooking non-potentially-constant bits in variable initializer
because we didn't walk into DECL_INITIAL.

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1): Look into
	DECL_INITIAL.  Use location wrappers.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-local4.C: Expect error sooner.
	* g++.dg/cpp2a/consteval24.C: Likewise.
	* g++.dg/cpp2a/consteval7.C: Likewise.
	* g++.dg/cpp2a/inline-asm3.C: Likewise.