It caused building Subzero individually to fail. Instead I've added
a header which will get included when the submodule is not present
and generates an error. The LLVM dependencies were split off into a
separate project to make this work.

This reverts commit d5d886eb.

Change-Id: Ie80a02e20ac67d6a4e0984e1fbab84b880a03469
Reviewed-on: https://swiftshader-review.googlesource.com/7611Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>