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.
Touch configure to force clang's config.h.in reconfiguration on the build servers

Gutted that this is needed, folks who care about keeping the legacy build
system alive should sort this out already.