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

[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 is a re-application of r190556 and r190808. This changes the interface
of GetSVN.cmake. Clang change to follow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222391 91177308-0d34-0410-b5e6-96231b3b80d8