libcpp: preprocessor speedup patches from mainline.

libcpp: preprocessor speedup patches from mainline.

* lex.c (_cpp_clean_line): Add uses of __builtin_expect.  Don't
look backward at the end of the line unless we saw a backslash.

* 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.

Obtained from:	gcc 4.3 (rev. 120263, 124929 ; GPLv2)
MFC after:	3 weeks