Add support for named values in the parser.

Summary: Add support for named values in the parser.

Reviewers: pcc

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D3276

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206176 91177308-0d34-0410-b5e6-96231b3b80d8