This commit updates our manifest generation for rustup to filter out any
components/extensions which are actually missing. This is intended to help
mitigate rust-lang#49462 by making the manifests reflect reality, that many targets now
are missing a `rust-docs` component rather than requiring it exists.

Fixes rust-lang#50595.

This bug currently affects stable. Why I think we can go for hard error:

- It will in stable for at most one cycle and there is no legitimate
reason to abuse it, nor any known uses in the wild.

- It only affects `bin` crates (which have a `main`), so there is
little practical difference between a hard error or a deny lint, both
are a one line fix.

The fix was to just unshadow a variable. Thanks @nikomatsakis for the
mentoring!

r? @nikomatsakis

Fix rust-lang#50811

Make sure the float comparison output is consistent with the expected behavior when NaN is involved.

----

Note: This PR is a **BREAKING CHANGE**. If you have used `>` or `>=` to compare floats, and make the result as the length of a fixed array type, like:

```rust
use std::f64::NAN;
let x: [u8; (NAN > NAN) as usize] = [1];
```

then the code will no longer compile. Previously, all float comparison involving NaN will just return "Greater", i.e. `NAN > NAN` would wrongly return `true` during const evaluation. If you need to retain the old behavior (why), you may replace `a > b` with `a != a || b != b || a > b`.

A high impact bug because a lot of common traits use a `Self`
substitution by default. Should be backported to beta.

There was a check for this which wasn't catching all cases, it was made
more robust.

Fixes rust-lang#49376
Fixes rust-lang#50626

r? @petrochenkov