* cppfiles.c (lookup_include_file): Rename to open_file.
	Always create a splay tree value, even on syscall failures.
	Negative entries indicated by fd == -2.
	Re-open files closed in the meantime.
	(_cpp_fake_include): Create a negative splay tree entry.
	(find_include_file, cpp_read_file): Update for function name
	change.
	* gcc.gd/cpp/mi4.c: Testcase.

From-SVN: r36564