libcpp: Fix _Pragma expansion [PR102409]

Both #pragma and _Pragma ended up as CPP_PRAGMA. Presumably since
r131819 (2008, GCC 4.3) for PR34692, pragmas are not expanded in
macro arguments but are output as is before. From the old bug report,
that was to fix usage like
  FOO (
    #pragma GCC diagnostic
  )
However, that change also affected _Pragma such that
  BAR (
    "1";
    _Pragma("omp ..."); )
yielded
  #pragma omp ...
followed by what BAR expanded too, possibly including '"1";'.

This commit adds a flag, PRAGMA_OP, to tokens to make the two
distinguishable - and include again _Pragma in the expanded arguments.

libcpp/ChangeLog:

	PR c++/102409
	* directives.c (destringize_and_run): Add PRAGMA_OP to the
	CPP_PRAGMA token's flags to mark is as coming from _Pragma.
	* include/cpplib.h (PRAGMA_OP): #define, to be used with token flags.
	* macro.c (collect_args): Only handle CPP_PRAGMA special if PRAGMA_OP
	is set.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/pragma-1.c: New test.
	* c-c++-common/gomp/pragma-2.c: New test.

libcpp/include/cpplib.h

@@ -198,6 +198,7 @@ struct GTY(()) cpp_string {
 				    operator, or before this token
 				    after a # operator.  */
 #define NO_EXPAND	(1 << 10) /* Do not macro-expand this token.  */
+#define PRAGMA_OP	(1 << 11) /* _Pragma token.  */
 
 /* Specify which field, if any, of the cpp_token union is used.  */