preprocessor: Don't register pragmas in directives-only mode [PR108244]

preprocessor: Don't register pragmas in directives-only mode [PR108244]

libcpp's directives-only mode does not expect deferred pragmas to be
registered, but to date the c-family registration process has not checked for
this case. That issue became more visible since r13-1544, which added the
commonly used GCC diagnostic pragmas to the set of those registered in
preprocessing modes. Fix it by checking for directives-only mode in
c-family/c-pragma.cc.

gcc/c-family/ChangeLog:

	PR preprocessor/108244
	* c-pragma.cc (c_register_pragma_1): Don't attempt to register any
	deferred pragmas if -fdirectives-only.
	(init_pragma): Likewise.

gcc/testsuite/ChangeLog:

	* c-c++-common/cpp/pr108244-1.c: New test.
	* c-c++-common/cpp/pr108244-2.c: New test.
	* c-c++-common/gomp/pr108244-3.c: New test.