[LLDB] Remove the Xcode project

Finally, after a lot of hard work from a bunch of people, we're in a
state where we can unify LLDB's build system.

This patch removes the hand-maintained Xcode project in favor of using
CMake in combination with the Xcode generator. Going forward, we want to
focus our efforts on improving the generated Xcode project.

Differential revision: https://reviews.llvm.org/D65109

llvm-svn: 366739