Makefile.rules: Use a shared clang module cache directory.

Just to be safe, up until now each test used its own Clang module
cache directory. Since the compiler within one testsuite doesn't
change it is just as safe to share a clang module directory inside the
LLDB test build directory. This saves us from compiling tens of
gigabytes of redundant Darwin and Foundation .pcm files and also
speeds up running the test suite quite significantly.

rdar://problem/36002081

Differential Revision: https://reviews.llvm.org/D54601

llvm-svn: 347056