Transform files in a compilation database if no sources are provided.

Compile all files in a compilation database or a subset determined by a
sub-directory so you  don't have to specify them manually.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@190630 91177308-0d34-0410-b5e6-96231b3b80d8