internal.h (struct cpp_reader): Add new fields: nonexistent_file_hash and nonexistent_file_ob.

	* internal.h (struct cpp_reader): Add new fields:
	nonexistent_file_hash and nonexistent_file_ob.
	* files.c: Include "obstack.h".
	(find_file_in_dir): Before trying to open the file, look up the
	path name in the hash table of nonexistent files.  After failing
	to open the file, add the path name to the hash table.
	(_cpp_find_file): Cache the results of looking up the file name
	starting with the quote and bracket chain heads, if we can.
	(nonexistent_file_hash_eq): New static function.
	(_cpp_init_files): Initialize pfile->nonexistent_file_hash and
	pfile->nonexistent_file_ob.
	(_cpp_cleanup_files): Free pfile->nonexistent_file_hash and
	pfile->nonexistent_file_ob.

From-SVN: r124929