Splitting up rustc_typeck::check::fn_ctxt::FnCtxt

[Nicholas-Baron]

Branching this from #60302.

Originally, mod.rs for rustc_typeck::check was over 6,000 lines. I was able to push that to around 1,100 lines (PR #76906).

However, in the process, I found that FnCtxt was rather long, with 13 fields in the struct and some rather long impls attached. The sum total is 3,200 lines, which effectively means that the // ignore-tidy-filelength comment was moved from mod.rs to fn_ctxt.rs.

The goal of this issue is to

1. Make fn_ctxt.rs smaller, at minimum less than 3,000 lines by
2. Moving some of its functionality into "sub-structs" that will be in separate files

Unknowns to solve:

• How do the 13 fields group into nice sub-structs
• What to name the above mentioned sub-structs
• How to achieve this refactoring without changing the interface of FnCtxt
  Solution: Cannot do

[Nicholas-Baron]

@rustbot modify labels to +T-compiler and +C-cleanup.

[Nicholas-Baron]

The third unknown is probably impossible. All fields (except err_count_on_creation) can be accessed from outside the file (pub(super)).

So the interface must change.

[Nicholas-Baron]

@rustbot modify labels to +E-needs-mentor.