Make LLDB's clang module cache path customizable

This patch makes LLDB's clang module cache path customizable via
settings set target.clang-modules-cache-path <path> and uses it in the
LLDB testsuite to reuse the same location inside the build directory
for LLDB and clang.

Differential Revision: https://reviews.llvm.org/D43099

llvm-svn: 324775