Rework Sema code completion interface.

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@87099 91177308-0d34-0410-b5e6-96231b3b80d8