Rework Sema code completion interface.

 - Provide Sema in callbacks, instead of requiring it in constructor. This
   eliminates the need for a factory function. Clients now just pass the object
   to consume the results in directly.

 - CodeCompleteConsumer is cheap to construct, so building it whenever we are
   doing code completion is reasonable.

Doug, please review.

llvm-svn: 87099