Rollup merge of #60928 - TheSirC:fix/60229, r=eddyb

Changes the type `mir::Mir` into `mir::Body`

Fixes part 1 of #60229 (previously attempted in #60242).

I stumbled upon the issue and it seems that the previous attempt at solving it was not merged. This is a second try more up-to-date.

The commit should have changed comments as well.
At the time of writting, it passes the tidy and check tool.

diff --cc src/librustc/mir/interpret/mod.rs
Simple merge

diff --cc src/librustc/mir/mod.rs
Simple merge

diff --cc src/librustc/ty/context.rs
Simple merge

diff --cc src/librustc_mir/borrow_check/nll/type_check/mod.rs
Simple merge

diff --cc src/librustc_mir/const_eval.rs
Simple merge

diff --cc src/librustc_mir/transform/const_prop.rs
Simple merge