[test] Fix various module cache bugs and inconsistencies

Currently, lit tests don't set neither the module cache for building
inferiors nor the module cache used by lldb when running tests.
Furthermore, we have several places where we rely on the path to the
module cache being always the same, rather than passing the correct
value around. This makes it hard to specify a different module cache
path when debugging a a test.

This patch reworks how we determine and pass around the module cache
paths and fixes the omission on the lit side. It also adds a sanity
check to the lit and dotest suites.

Differential revision: https://reviews.llvm.org/D66966

llvm-svn: 370394