Support named values in the autocomplete feature.

Summary:
This includes:
 - Passing a Sema to completeExpression to allow for named values in the
   expression.
 - Passing a map of names to values to the parser.
 - Update the Sema interface to include completion for matchers.
 - Change the parser to use the Sema for completion, instead of going
   directly to Registry.

Reviewers: pcc

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D3509