Compilation database test: don't try to output to CWD

Summary:
Write output from compilation database test to %T rather than the working dir.
Sometimes CWD isn't writable!
Also specify no-canonical-prefixes so that clang has 'clang' in the name.

Reviewers: bkramer

Subscribers: joerg, cfe-commits

Differential Revision: https://reviews.llvm.org/D27504

llvm-svn: 288892