Add immediate potential_constant_expression variants.

	* constexpr.c (potential_constant_expression_1): Add "now" parm.
	(is_constant_expression, require_constant_expression): New.
	(is_static_init_expression, is_nondependent_constant_expression)
	(is_nondependent_static_init_expression): Drop "potential".
	* except.c (build_must_not_throw_expr): Do type conversion on
	value-dependent argument.
	* pt.c, semantics.c, typeck2.c: Use variants without "potential".

From-SVN: r251423