[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.
Constify a few IdentifierInfo * parameters.

No functionality change.
[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. We avoid having to regenerate SVNVersion.inc
every time by marking it as dependent on Git's reflog or SVN's entries file.

This should end most of 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.

Based on r190557. Depends on LLVM r222391.