Completely refactor the structuring of unittest CMake files to match the

Makefiles, the CMake files in every other part of the LLVM tree, and
sanity.

This should also restore the output tree structure of all the unit
tests, sorry for breaking that, and thanks for letting me know.

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:

- No more weird directory stripping in the unittest macro, allowing it
  to be used more readily in other projects.
- No more directory prefixes on all the source files.
- Allows correct and precise use of LLVM's per-directory dependency
  system.
- Allows use of the checking logic for source files that have not been
  added to the CMake build. This uncovered a file being skipped with
  CMake in LLVM and one in Clang's unit tests.
- Makes Specifying conditional compilation or other custom logic for JIT
  tests easier.

It did require adding the concept of an explicit 'optional' source file
to the CMake build so that the missing-file check can skip cases where
the file is *supposed* to be missing. =]

This is another chunk of refactoring the CMake build in order to make it
usable for other clients like CompilerRT / ASan / TSan.

Note that this is interdependent with a Clang CMake change.

llvm-svn: 158909