re PR c++/88977 (__builtin_is_constant_evaluated() as function template argument causes substitution failure)

	PR c++/88977
	* pt.c (convert_nontype_argument): Pass true as manifestly_const_eval
	to maybe_constant_value calls.

	* g++.dg/cpp2a/is-constant-evaluated7.C: New test.

From-SVN: r268780