Add a -D flag to FileCheck to define variables

Summary:
This makes it very easy to test files that only differ in a constant
value somewhere in the test case.

Reviewers: jlebar, hfinkel, chandlerc, probinson

Reviewed By: probinson

Subscribers: probinson, llvm-commits

Differential Revision: https://reviews.llvm.org/D39629

llvm-svn: 317572