Add instructions for building LLDB on Mac OS X with CMake

Summary:
There were previously no instructions for building LLDB on Mac OS X
with CMake. It's sufficiently close to building on Linux/FreeBSD/NetBSD that a
well-motivated developer could figure out the steps themselves. However, having
an explicit guide is better and can provide Mac OS X specific configurations
(e.g. LLDB_BUILD_FRAMEWORK).

Reviewers: labath, clayborg

Subscribers: emaste, krytarowski

Differential Revision: https://reviews.llvm.org/D50362

llvm-svn: 339155