Revert "Revert "Devise a package-private means to determine the LLVM version string""

We probably just need to touch LLVM's configure this time to work around the
totally inadequate Makefile build server integration.

This reverts commit r210314.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210320 91177308-0d34-0410-b5e6-96231b3b80d8