Driver: Consolidate the logic for naming the module crashdump cache

List the module cache we use for crashdumps as a tempfile. This
simplifies how we pick up this directory when generating the actual
crash diagnostic and removes some duplicate logic.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@220241 91177308-0d34-0410-b5e6-96231b3b80d8