The fundamental change is to put a CMakeLists.txt file in the unittest
directory, with a single test binary produced from it. This has several
advantages.

Among other fundamental advantages, we start to get the checking logic
for when a file is missing from the CMake build, and this caught one
missing file already! More fun details in the LLVM commit corresponding
to this one.

Note that the LLVM commit and this one most both be applied, or neither.
Sorry for any skew issues.

llvm-svn: 158910