Fix a marvelous chained AST writing bug, where we end up with the

following amusing sequence:
  - AST writing schedules writing a type X* that it had never seen
  before
  - AST writing starts writing another declaration, ends up
  deserializing X* from a prior AST file. Now we have two type IDs for
  the same type!
  - AST writer tries to write X*. It only has the lower-numbered ID
  from the the prior AST file, so references to the higher-numbered ID
  that was scheduled for writing go off into lalaland.

To fix this, keep the higher-numbered ID so we end up writing the type
twice. Since this issue occurs so rarely, and type records are
generally rather small, I deemed this better than the alternative: to
keep a separate mapping from the higher-numbered IDs to the
lower-numbered IDs, which we would end up having to check whenever we
want to deserialize any type.

Fixes <rdar://problem/8511624>, I think.

llvm-svn: 115647