Add parsing of 'foo@local".

Summary:
Currently, it supports generating, but not parsing, this expression.
Test added as well.

Test Plan: New test added, no regressions due to this.

Reviewers: hfinkel

Reviewed By: hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6672

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224415 91177308-0d34-0410-b5e6-96231b3b80d8