Auto merge of #21731 - syniuhin:add-llvm-to-macos-deps, r=asajeffrey

Add llvm to macOS required dependencies in docs

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@gterzian described it [in their comment](https://github.com/servo/servo/issues/21498#issuecomment-415698332) pretty well:

> latest "command line tools" that come with it(and include Apple's version of llvm), it might be that the current default Mac llvm is too old to compile the latest mozjs version(it appeared that after the OS update, no version of clang/llvm was installed until I downloaded the "new" command-line tools for the new OS)

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #21498 (github issue number if applicable).

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- [X] These changes do not require tests because it's a change to markdown file.

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