This commit adds a new lint - `improper_ctypes_definitions` - which
functions identically to `improper_ctypes`, but on `extern "C" fn`
definitions (as opposed to `improper_ctypes`'s `extern "C" {}`
declarations).
Signed-off-by: NDavid Wood <david@davidtw.co>

This reverts commit 3f0e1647, reversing
changes made to 61a551b4.

Add `#![allow(..)]` as necessary to get re-migrated run-pass tests compiling with clean stderr again.

Most were added mechanically.

Fix #54047

(I may have accidentally added it to some auxilliary crates as well;
my emacs-macro-based methodology was pretty crude.)

https://github.com/rust-lang/rust/issues/26997

Struct variant field visibility is now inherited. Remove `pub` keywords
from declarations.

Closes #18641

[breaking-change]

Closes #2569