[llvm] Include utils/unittest before projects and runtimes

Include utils/unittest before projects and runtimes so that downstream
projects can check for the existence of the llvm_gtest target. This is
motivated by 9c6c4d67 which fixes the stand-alone build
configuration where LLVM_MAIN_SRC_DIR does not exist.

Differential revision: https://reviews.llvm.org/D124314