[WIP] Run the unused variables lint on the MIR

[ecstatic-morse]

Resolves #51003.

This removes the HIR liveness pass that is currently responsible for the unused variables lint. The idea is that this will reduce the overall difficulty of updating the HIR. Unfortunately, because we are moving this analysis further away from the AST, we need to do some tricks to map MIR locals back the variables they are tied to. One source of friction is bindings in match patterns, which are simultaneously associated with two distinct locals, one in the guard and one in the match arm body.

Due to a shortcoming of the incremental tests, I cannot add a HirId to VarBindingForm. As a result, we are storing way too much stuff in VarBindingForm, and properly handling multiple bindings with the same name (e.g., Ok(x) | Err(x)) will require more still. I'd like to fix the underlying issue and start storing HirIds in the VarBindingForm, but I think this will require changing the problematic tests? Until we can store a HirId on VarBindingForm, I likely won't prioritize full diagnostic parity with the HIR liveness pass, since my current approach seems clearly wrong.

I don't think it's clear that this approach will be easier to maintain than the HIR liveness analysis. I'm posting this draft so that people can weigh in on that question. If it is resolved in the affirmative, I'll continue working on this, but until then it's not ready for a detailed review.

[rust-highfive]

r? @petrochenkov

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[ecstatic-morse]

Forgot the r? @ghost. Sorry

[ecstatic-morse]

Some more TODOs for myself:

• Add FakeRead::ForLetUnderscore to all places where we do no-op matches. For example, let () = expr;. Possibly change the name?
• Investigate behavior for async fn parameters, which seem to always be used by the internal generator.

[rust-highfive]

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##[section]Starting: Linux x86_64-gnu-llvm-8
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Step 5/8 : ENV RUST_CONFIGURE_ARGS       --build=x86_64-unknown-linux-gnu       --llvm-root=/usr/lib/llvm-8       --enable-llvm-link-shared       --set rust.thin-lto-import-instr-limit=10
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---
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   Compiling rustc_trait_selection v0.0.0 (/checkout/src/librustc_trait_selection)
error: variable `convert_to_ambiguous` is assigned to, but never used
   |
90 |         let convert_to_ambiguous;
   |             ^^^^^^^^^^^^^^^^^^^^
   |
   |
   = note: `-D unused-variables` implied by `-D warnings`
   = note: consider using `_convert_to_ambiguous` instead
error: aborting due to previous error

error: could not compile `rustc_trait_selection`.

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  local time: Wed May 13 17:02:46 UTC 2020
  network time: Wed, 13 May 2020 17:02:46 GMT
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##[error]Bash exited with code '1'.
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Author       : Microsoft
Author       : Microsoft
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[bors]

☔ The latest upstream changes (presumably #69171) made this pull request unmergeable. Please resolve the merge conflicts.

[ecstatic-morse]

This will require a lot more work, and I'm still not sure if it's the correct approach. I don't anticipate I'll get back to it anytime soon.