c++: two-parameter version of cxx_constant_value

Since some callers need the complain parameter but not the object
parameter, let's introduce and use an overload of cxx_constant_value
that omits the latter.

gcc/cp/ChangeLog:

	* cp-tree.h (cxx_constant_value): Define two-parameter version
	that omits the object parameter.
	* decl.cc (build_explicit_specifier): Omit NULL_TREE object
	argument to cxx_constant_value.
	* except.cc (build_noexcept_spec): Likewise.
	* pt.cc (expand_integer_pack): Likewise.
	(fold_targs_r): Likewise.
	* semantics.cc (finish_if_stmt_cond): Likewise.