Auto merge of #85788 - rylev:force-warns, r=nikomatsakis

Support for force-warns

Implements https://github.com/rust-lang/rust/issues/85512.

This PR adds a new command line option `force-warns` which will force the provided lints to warn even if they are allowed by some other mechanism such as `#![allow(warnings)]`.

Some remaining issues:
* https://github.com/rust-lang/rust/issues/85512 mentions that `force-warns` should also be capable of taking lint groups instead of individual lints. This is not implemented.
* If a lint has a higher warning level than `warn`, this will cause that lint to warn instead. We probably want to allow the lint to error if it is set to a higher lint and is not allowed somewhere else.
* One test is currently ignored because it's not working - when a deny-by-default lint is allowed, it does not currently warn under `force-warns`. I'm not sure why, but I wanted to get this in before the weekend.

r? `@nikomatsakis`

diff --cc compiler/rustc_lint/src/context.rs
Simple merge

diff --cc compiler/rustc_session/src/config.rs
Simple merge

diff --cc compiler/rustc_session/src/options.rs
Simple merge

diff --cc src/librustdoc/config.rs
Simple merge

diff --cc src/librustdoc/lib.rs
Simple merge