c++: cp_parser_constant_expression cleanups

It's pointless to call *_rvalue_constant_expression when we're not using
the result.  Also apply some drive-by cleanups.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_constant_expression): Allow non_constant_p to be
	nullptr even when allow_non_constant_p is true.  Don't call
	_rvalue_constant_expression when not necessary.  Move local variable
	declarations closer to their first use.
	(cp_parser_static_assert): Don't pass a dummy down to
	cp_parser_constant_expression.