Implement P1902R1, Missing feature-test macros 2017-2019.

Implement P1902R1, Missing feature-test macros 2017-2019.

	* c-cppbuiltin.c (c_cpp_builtins): Bump __cpp_init_captures
	and __cpp_generic_lambdas for -std=c++2a.  Define
	__cpp_designated_initializers, __cpp_constexpr_in_decltype and
	__cpp_consteval for -std=c++2a.  Remove a FIXME comment about
	__cpp_concepts for -std=c++2a.

	* g++.dg/cpp1z/feat-cxx1z.C: Only compile with -std=c++17.
	* g++.dg/cpp2a/feat-cxx2a.C: Adjust for P1902R1 changes.
	* g++.dg/cpp2a/desig15.C: New test.
	* g++.dg/cpp2a/lambda-pack-init3.C: New test.
	* g++.dg/cpp2a/lambda-generic6.C: New test.
	* g++.dg/cpp2a/consteval15.C: New test.

From-SVN: r278628