Modified --test-pickling to perform an actual cross-check of serialized ASTs:

(1) Parsed ASTs are pretty-printed to a text file.
(2) The ASTs are serialized to disk.
(3) The ASTs are deserialized from disk.
(4) The deserialized ASTs are pretty-printed to a text file.
(5) The two pretty-printed files are compared. If they are different, the test
    fails.

llvm-svn: 44411