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

This will unbreak clang vendor builds as a follow-up to r210238, now that we
can't poke into LLVM's private config.h (nor should the string be exposed by
llvm-config.h).

This hopefully removes for good the last include of LLVM's config.h.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210313 91177308-0d34-0410-b5e6-96231b3b80d8