Make the clang module cache setting available without a target

It turns out that setting the clang module cache after LLDB has a
Target can be too late. In particular, the Swift language plugin needs
to know the setting without having access to a Target. This patch
moves the setting into the *LLDB* module cache, where it is a global
setting that is available before any Target is created and more
importantly, is shared between all Targets.

rdar://problem/37944432

Differential Revision: https://reviews.llvm.org/D43984

llvm-svn: 326628