[PCH] Make the new PCH format (control block) backwards compatible and

don't crash when loading a PCH with the older format.

The introduction of the control block broke compatibility with PCHs from
older versions. This patch allows loading (and rejecting) PCHs from an older
version and allows newer PCHs to be rejected from older clang versions as well.

rdar://12821386

llvm-svn: 170150