Use a shared module cache directory for LLDB.

This saves about 3 redundant gigabytes from the Objective-C test build
directories. Tests that must do unsavory things with the LLDB clang
module cache, already specify a per-test module cache in their .py
test instructions.

<rdar://problem/36002081>

Differential Revision: https://reviews.llvm.org/D54602

llvm-svn: 347057