This patch lets LLDB build as an LLVM subproject. LLDB is not built in

This patch lets LLDB build as an LLVM subproject.  LLDB is not built in
parallel with the rest of the tools directory as it depends on Clang.

This patch was first applied in r125956 and subsequently reverted in
r125964 as it broke in-tree builds.  Makefile.rules was fixed up in
r126070 to handle missing optional directories for the in-tree case,
so it should be safe now to bring this patch back in.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126071 91177308-0d34-0410-b5e6-96231b3b80d8