Dual docs for llvm usage: cli and Xcode

[springmeyer]

XCode/Apple is driving the development of llvm tools. These tools can and do work on the command line on both os x and linux. So we should prioritize documenting cli usage in a cross platform way. However users of XCode, notably for mobile development, can and should prefer to use features of llvm like the sanitizers from within Xcode GUI (with the caveat that at the time of writing Xcode only supports the address sanitizer and not thread, memory, undefined).

As such our docs ideally should target the three ways you'd use llvm:

• cli on linux (installed with mason)
• cli on osx (installed with mason or recent Xcode command line tools - identical behavior to linux except for missing sanitizers)
• XCode UI on OS X for macOS and iOS development (installed with XCode)