Autotools has the same include guard for both Clang and LLVM's config.h.

Shuffling order causes the wrong one to win.

CMake didn't exhibit this problem because Clang's has *no* guards.

I'll fix this properly tomorrow when Eric and I can check both build
systems and get them to DTRT, but for now unbreak some bots by hoisting
this header.

llvm-svn: 169260