This allows building a rustc capable of running the frontend without any
backend present. While this may not seem all that useful, it allows
running the frontend of rustc to report errors or running miri to
interpret a program without any backend present. This is useful when you
are trying to say run miri in the browser as upstream LLVM can't be
compiled for wasm yet. Or to run rustc itself in miri like I did a while
ago and caught some UB.
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