HostInfoMacOSX: Share the clang resource directory with Swift.

Inside Xcode and in Xcode toolchains LLDB is always in lockstep
with the Swift compiler, so it can reuse its Clang resource
directory. This allows LLDB and the Swift compiler to share the
same Clang module cache.

rdar://problem/40039633

Differential Revision: https://reviews.llvm.org/D46736

llvm-svn: 332111