Fix getCompiler in unit testing framework on compiler symlinks

Summary:
Checks using the result of getCompiler() will fail to identify the compiler
correctly if CC is a symlink path (ie /usr/bin/cc).

Reviewers: zturner, emaste

Subscribers: llvm-commits, sas

Differential Revision: http://reviews.llvm.org/D16488

Change by Francis Ricci <fjricci@fb.com>

llvm-svn: 259433