re PR c++/86524 (std::less with pointer arguments not usable in static_assert in constexpr function)

	PR c++/86524
	PR c++/88446
	* cp-tree.h (cp_fold_maybe_rvalue, cp_fold_rvalue): Declare.
	(fold_non_dependent_expr): Add manifestly_const_eval argument.
	* constexpr.c (cxx_eval_builtin_function_call): Evaluate
	__builtin_constant_p if ctx->manifestly_const_eval even in constexpr
	functions.  Don't reuse dummy{1,2} vars between different arguments.
	Use cp_fold_rvalue instead of cp_fully_fold.  Fix comment typo.
	(fold_non_dependent_expr): Add manifestly_const_eval argument, pass
	it through to cxx_eval_outermost_constant_expr and
	maybe_constant_value.
	* cp-gimplify.c (cp_fold_maybe_rvalue, cp_fold_rvalue): No longer
	static.
	* semantics.c (finish_static_assert): Call fold_non_dependent_expr
	with true as manifestly_const_eval.

	* g++.dg/cpp1y/constexpr-86524.C: New test.
	* g++.dg/cpp2a/is-constant-evaluated4.C: New test.
	* g++.dg/cpp2a/is-constant-evaluated5.C: New test.
	* g++.dg/cpp2a/is-constant-evaluated6.C: New test.

From-SVN: r267341