[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190556 91177308-0d34-0410-b5e6-96231b3b80d8