Dual docs for llvm usage: cli and Xcode

还没有人认领这个 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)

未指定任何文件或测试。首先定位现有的 LLVM 文档及其 CLI 和 Xcode 入口点，然后梳理所请求的 Linux CLI、macOS CLI 和 Xcode UI 工作流。完成的标准是跨平台记录这三种使用路径及其 sanitizer 限制。

由索引模型根据 Issue 内容生成。