[CMake] Update GetSVN.cmake to use LLVM version control helper scripts.

This allows the logic to work with Git, and also uses the variable names
to match what Clang is actually looking for.

This changes the interface of GetSVN.cmake. Clang change to follow.
[CMake] Always include the Clang repo version, just like the autoconf build.

Now that LLVM's helper script GetSVN.cmake actually works consistently,
there's no reason not to use it. This does mean that the clangBasic target
is potentially always dirty, because CMake-generated projects do not
necessarily recalculate dependencies after running each target.

This should end the issues of the AST format changing and breaking old
module files; CMake-Clang should now detect that the version changed just
like Autoconf-Clang has.