Fix driver tests when LLVM_ENABLE_PER_TARGET_RUNTIME_DIR is ON
Some Driver tests relied on the default resource direcory having per-os per-arch
subdirectory layout, and when clang is built with -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON,
those test fail, because clang by default assumes per-target subdirectories.
Explicitly set -resource-dir flag to point to a tree with per-os per-arch layout.
Differential Revision: https://reviews.llvm.org/D66981
Patch by Sergej Jaskiewicz <email@example.com>.