Auto merge of #67032 - cjgillot:hirene, r=Zoxc

Allocate HIR on an arena 4/4

This is the fourth and last PR in the series started by #66931, #66936 and #66942.

The last commits should compile on their own.
The difference with the previous PR is given by https://github.com/cjgillot/rust/compare/hirene-ty...hirene

A few more cleanups may be necessary, please tell me.

r? @eddyb like the other
cc @Zoxc

diff --cc src/librustc_mir/borrow_check/diagnostics/region_name.rs
Simple merge