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

llvm-svn: 205419