Unit test support for Clang extra tools

Added support to CMake and autoconf for unit tests in clang-tools-extra. A
dummy test exists for now until more meaningful tests can be written.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@178661 91177308-0d34-0410-b5e6-96231b3b80d8