libcpp: Fix ICE on #include after a line marker directive [PR61474]

As noted in the PR, GCC will segfault if a file name is first seen in a
linemarker directive, and then later seen in a normal #include.  This is
because the fake include process adds the file to the cache with a null PATH
member. The normal #include finds this file in the cache and then attempts
to use the null PATH.  Resolve by adding the file to the cache with a unique
starting directory, so that the fake entry will only be found by a
subsequent fake include, not by a real one.

libcpp/ChangeLog:

	PR preprocessor/61474
	* files.cc (_cpp_find_file): Set DONT_READ to TRUE for fake
	include files.
	(_cpp_fake_include): Pass a unique cpp_dir* address so
	the fake file will not be found when looked up for real.

gcc/testsuite/ChangeLog:

	PR preprocessor/61474
	* c-c++-common/cpp/pr61474-2.h: New test.
	* c-c++-common/cpp/pr61474.c: New test.
	* c-c++-common/cpp/pr61474.h: New test.