[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
gold-plugin: Fix typo in error message

Spotted while reading code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222395 91177308-0d34-0410-b5e6-96231b3b80d8
[Hexagon] Adding A2_or instruction with IR selection pattern and test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222396 91177308-0d34-0410-b5e6-96231b3b80d8