[PCH] Overhaul how preprocessed entities are [de]serialized.

-Use an array of offsets for all preprocessed entities
-Get rid of the separate array of offsets for just macro definitions;
 for references to macro definitions use an index inside the preprocessed
 entities array.
-Deserialize each preprocessed entity lazily, at first request; not in bulk.

Paves the way for binary searching of preprocessed entities that will offer
efficiency and will simplify things on the libclang side a lot.

llvm-svn: 139809