Add support for named values in the parser.

Add support for named values in the parser.

Summary:
Add support for named values in the parser.
This allows injection of arbitrary constants using a custom Sema object.
Completions are not supported right now.

Will be used by clang_query to support the 'let' command.
Usage example:
  clang_query> let unique_ptr recordDecl(hasName("unique_ptr"))
  clang_query> match varDecl(hasType(unique_ptr))

Reviewers: klimek, pcc

CC: cfe-commits, klimek
Differential Revision: http://llvm-reviews.chandlerc.com/D3229

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205419 91177308-0d34-0410-b5e6-96231b3b80d8