Introduce -DLLVM_USE_SANITIZER=Undefined CMake option to build UBSan-ified version of LLVM/Clang.

I've fixed most of the simple bugs and currently "check-llvm" test suite
has 26 failures, and "check-clang" suite has 5 failures.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216701 91177308-0d34-0410-b5e6-96231b3b80d8