Auto merge of #155223 - teor2345:fndef-refactor, r=mati865

Refactor FnDecl and FnSig non-type fields into a new wrapper type





#### Why this Refactor?

This PR is part of an initial cleanup for the [arg splat experiment](https://github.com/rust-lang/rust/issues/153629), but it's a useful refactor by itself.

It refactors the non-type fields of `FnDecl`, `FnSig`, and `FnHeader` into a new packed wrapper types, based on this comment in the `splat` experiment PR:
https://github.com/rust-lang/rust/pull/153697#discussion_r3004637413

It also refactors some common `FnSig` creation settings into their own methods. I did this instead of creating a struct with defaults.

#### Relationship to `splat` Experiment

I don't think we can use functional struct updates (`..default()`) to create `FnDecl` and `FnSig`, because we need the bit-packing for the `splat` experiment.

Bit-packing will avoid breaking "type is small" assertions for commonly used types when `splat` is added.
This PR packs these types:
- ExternAbi: enum + `unwind` variants (38) -> 6 bits
- ImplicitSelfKind: enum variants (5) -> 3 bits
- lifetime_elision_allowed, safety, c_variadic: bool -> 1 bit

#### Minor Changes

Fixes some typos, and applies rustfmt to clippy files that got skipped somehow.