libcpp: Fix _Pragma("GCC system_header") [PR114436]

_Pragma("GCC system_header") currently takes effect only partially. It does
succeed in updating the line_map, so that checks like in_system_header_at()
return correctly, but it does not update pfile->buffer->sysp.  One result is
that a subsequent #include does not set up the system header state properly
for the newly included file, as pointed out in the PR. Fix by propagating
the new system header state back to the buffer after processing the pragma.

libcpp/ChangeLog:

	PR preprocessor/114436
	* directives.cc (destringize_and_run): If the _Pragma changed the
	buffer system header state (e.g. because it was "GCC
	system_header"), propagate that change back to the actual buffer
	too.

gcc/testsuite/ChangeLog:

	PR preprocessor/114436
	* c-c++-common/cpp/pragma-system-header-1.h: New test.
	* c-c++-common/cpp/pragma-system-header-2.h: New test.
	* c-c++-common/cpp/pragma-system-header.c: New test.