[CrashReproducer] Provide a clean dir path for -fmodules-cache-path

The most common workflow with module reproducers involves deleting the
module cache before running the script. This happens because leftovers
from the crash are present in the cache and could trigger unrelated and
confusing errors, misleading from the initial reproduction intent.
Change this to point to a clean path but leave the leftovers untouched.

rdar://problem/28655070

llvm-svn: 289176