[test] Use a different module cache for Shell and API tests.
Before the test reorganization, everything was part of a single test
suite with a single module cache. Now that things are properly separated
this is no longer the case. Only the shell tests inherited the logic to
properly configure and wipe the module caches. This patch adds that
logic back for the API tests. While doing so, I noticed that we were
configuring a Clang module cache in CMake, but weren't actually using it
from dotest.py. I included a fix for that in this patch as well.
Differential revision: https://reviews.llvm.org/D68755