Dual docs for llvm usage: cli and Xcode

還沒有人認領這個 Issue。

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)

未指定任何檔案或測試。首先定位現有的 LLVM 文件及其 CLI 和 Xcode 入口點，然後整理所要求的 Linux CLI、macOS CLI 和 Xcode UI 工作流程。完成的標準是跨平台記錄這三種使用路徑及其 sanitizer 限制。

由索引模型根據 Issue 內容生成。