Fix llvm-config support for CMake build-mode-style builds

At some point we and/or CMake changed our build-mode-style builds from
  $LLVM_OBJ_ROOT/bin/$CMAKE_CFG_INTDIR/
to
  $LLVM_OBJ_ROOT/$CMAKE_CFG_INTDIR/bin/
which is way easier to use. But no one updated llvm-config.

https://reviews.llvm.org/D66326

llvm-svn: 369129