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.

See also: D45604, D62469

Differential Revision: https://reviews.llvm.org/D66981

Patch by Sergej Jaskiewicz <jaskiewiczs@icloud.com>.

llvm-svn: 373565