libcpp: Fix _Pragma stringification [PR103165]

libcpp: Fix _Pragma stringification [PR103165]

As the testcase show, sometimes _Pragma is turned into CPP_PRAGMA
.. CPP_PRAGMA_EOL tokens, even when it might still need to be
stringized later on.  We are then ICEing because we don't handle
stringification of CPP_PRAGMA or CPP_PRAGMA_EOL, but trying to
reconstruct the exact tokens with exact spacing after it has been
lowered is very hard.  So, instead this patch ensures we don't
lower _Pragma during expand_arg calls, but only later when
cpp_get_token_1 is called outside of expand_arg.

2021-11-22  Jakub Jelinek  <jakub@redhat.com>
	    Tobias Burnus  <tobias@codesourcery.com>

	PR preprocessor/103165
libcpp/
	* internal.h (struct lexer_state): Add ignore__Pragma field.
	* macro.c (builtin_macro): Don't interpret _Pragma if
	pfile->state.ignore__Pragma.
	(expand_arg): Temporarily set pfile->state.ignore__Pragma to 1.
gcc/testsuite/
	* c-c++-common/gomp/pragma-3.c: New test.
	* c-c++-common/gomp/pragma-4.c: New test.
	* c-c++-common/gomp/pragma-5.c: New test.

Co-Authored-By: Tobias Burnus <tobias@codesourcery.com>