emacs: do not highlight `#foo]` as an attribute

part of #14347
emacs: highlight `#[foo = "bar"]` attributes

Setting `OVERRIDE` to `t` overrides the conflicting highlight Emacs
places on string literals.

Fix #14347
Updated deps
Platform configuration

FIXME: avoid using Xcode build and switch to a patched libuv
Runtime support for arm on iOS
Disable generating split-stack code

Allows to compile for archs which do not have (or have limited)
segmented stack support like embedded.
Basic iOS support
Better dylib skipping based on Alex Crichton code
Relicense shootout-pidigits.rs

Everyone agreed.

Related to #14248, close #14718
Relicense shootout-binarytrees.rs

Everyone agreed.

Related to #14248, close #14720
Cosmetic fixes & comments
There should be no metadata to read on iOS
librustc: Forbid enum-to-float casts.

Closes #14794.

If you're casting from an enum to a float, cast through an integer
first.

[breaking-change]
librustc: Remove useless code to handle `let _ = ...`

This code didn't do anything, but was a vestige of the old semantics for
`let _ = ...`.

Closes #10488. (As near as I can tell anyhow.)
getopts: format failure messages with `Show`.

This obsoletes the old `to_err_msg` method. Replace

    println!("Error: {}", failure.to_err_msg())

    let string = failure.to_err_msg();

with

    println!("Error: {}", failure)

    let string = failure.to_str();

[breaking-change]
getopts: derive Eq for types.
Make analyze_move_out_from use a loop rather than recursion

It will be simpler to make some of the changes that I need to make to
analyze_move_out if it uses a loop rather than recursion.
Make check_for_move_of_borrowed_path take an &LoanPath rather than an &Rc<LoanPath>

It doesn't actually need the Rc, and it reduces the net number of
pointer manipulations.
Make analyze_move_out_from more field-sensitive

Currently analyze_move_out_from checks all restrictions on all base
paths of the move path, but it only needs to check restrictions from
loans of the base paths, and can disregard restrictions from loans of
extensions of those base paths.
Make analyze_move_out_from take a BorrowKind

Currently analyze_move_out_from ignores the BorrowKind of loans, but the
same logic is useful when restricted to loans of specific borrow kinds.