This macro was renamed after it was added to the working draft, but we
never renamed it  in libstdc++. We haven't made a release with the old
macro name, so I see no need to keep it around.

	* include/std/functional (__cpp_lib_constexpr_invoke): Rename to
	__cpp_lib_constexpr_functional.
	* include/std/version (__cpp_lib_constexpr_invoke): Likewise.
	* testsuite/20_util/function_objects/invoke/constexpr.cc: Adjust.