Start factoring the on-disk records for an AST file into a control

block, which stores information about how the AST file to generated,
from the AST block, which stores the actual serialized AST. The
information in the control block should be enough to determine whether
the AST file is up-to-date and compatible with the current translation
unit, and reading it should not cause any side effects that aren't
easy to undo. That way, we can back out from an attempt to read an
incompatible or out-of-date AST file.

Note that there is still more factoring to do. In particular,
information about the source files used to generate the AST file
(along with their time stamps, sizes, etc.) still resides in the
source manager block. 

llvm-svn: 166166