Auto merge of #45283 - alexcrichton:used-mut-nodes, r=arielb1

rustc: Remove `used_mut_nodes` from `TyCtxt`

This updates the borrowck query to return a result, and this result is then used
to incrementally check for unused mutable nodes given sets of all the used
mutable nodes.

Closes #42384

diff --cc src/Cargo.lock
Simple merge

diff --cc src/librustc_borrowck/borrowck/mod.rs
Simple merge

diff --cc src/librustc_borrowck/lib.rs
index 11120d2e46f2af6ed09d7a316bfeeec19610908c,0a39dc1d8b5a2c6709e00b0177765c737693db4c..78aacd49f807dafafc8ae37f7f29db8ed6e81e27
--- 1/src/librustc_borrowck/lib.rs
--- 2/src/librustc_borrowck/lib.rs
+++ b/src/librustc_borrowck/lib.rs
@@@ -16,11 -16,13 +16,12 @@@
  #![allow(non_camel_case_types)]
  
  #![feature(quote)]
 -#![feature(rustc_diagnostic_macros)]
  
  #[macro_use] extern crate log;
 -#[macro_use] extern crate syntax;
 +extern crate syntax;
  extern crate syntax_pos;
  extern crate rustc_errors as errors;
+ extern crate rustc_back;
  
  // for "clarity", rename the graphviz crate to dot; graphviz within `borrowck`
  // refers to the borrowck-specific graphviz adapter traits.