c++: Make -Wunknown-pragmas controllable by #pragma GCC diagnostic [PR89038]

As noted on the PR, commit r13-1544, the fix for PR53431, did not handle
the specific case of -Wunknown-pragmas, because that warning is issued
during preprocessing, but not by libcpp directly (it comes from the
cb_def_pragma callback).  Address that by handling this pragma in
addition to libcpp pragmas during the early pragma handler.

gcc/c-family/ChangeLog:

	PR c++/89038
	* c-pragma.cc (handle_pragma_diagnostic_impl):  Handle
	-Wunknown-pragmas during early processing.

gcc/testsuite/ChangeLog:

	PR c++/89038
	* c-c++-common/cpp/Wunknown-pragmas-1.c: New test.