libcpp: Fix ICE on directive inside _Pragma() operator [PR67046]

get__Pragma_string() in directives.cc is responsible for lexing the parens
and the string argument from a _Pragma("...") operator. This function does
not handle the case when the closing paren is not on the same line as the
string; in that case, libcpp will by default reuse the token buffer it
previously used for the string, so that the string token returned by
get__Pragma_string() may be corrupted, as shown in the testcase. Fix using
the existing keep_tokens mechanism that temporarily disables the reuse of
token buffers.

libcpp/ChangeLog:

	PR preprocessor/67046
	* directives.cc (_cpp_do__Pragma): Increment pfile->keep_tokens to
	ensure the returned string token is valid.

gcc/testsuite/ChangeLog:

	PR preprocessor/67046
	* c-c++-common/cpp/pr67046.c: New test.