c++: NRVO, constexpr, lambda [PR118053]

c++: NRVO, constexpr, lambda [PR118053]

Here during constant evaluation we encounter a VAR_DECL with DECL_VALUE_EXPR
of the RESULT_DECL, where the latter has been adjusted for
pass-by-invisible-reference.  We already had the code to deal with this, we
just need to use it in the non-capture case of DECL_VALUE_EXPR as well.

	PR c++/118053

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression): Generalize
	DECL_VALUE_EXPR invisiref handling.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-lambda1.C: New test.