[CTU] Eliminate race condition in CTU lit tests

Summary:
We plan to introduce additional CTU related lit test. Since lit may run the
tests in parallel, it is not safe to use the same directory (%T) for these
tests. It is safe to use however test case specific directories (%t).

Reviewers: xazax.hun, a_sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Differential Revision: https://reviews.llvm.org/D55129

llvm-svn: 348587