rustdoc/clean: Fix lowering of fn params (fixes correctness & HIR vs. middle parity regressions)
**(0)** PR #136411 aimed to stop rendering unnamed params of fn ptr types as underscores in the common case (e.g., `fn(_: i32)` → `fn(i32)`) to make the rendered output stylistically more conventional.
**(0.a)** However, since the cleaning fn that the PR modified is also used for lowering the HIR params of foreign fns and required assoc fns in traits, it accidentally butchered the rendering of the latter two:
```rs
pub trait Trait { fn assoc_fn(_: i32); } // as well as (Rust 2015 only): fn assoc_fn(i32);
unsafe extern "C" { pub fn foreign_fn(_: i32); }
// Since 1.86 the fns above gets mis-rendered as:
pub fn assoc_fn(: i32) // <-- BUTCHERED
pub unsafe extern "C" fn foreign_fn(: i32) // <-- BUTCHERED
```
**(0.b)** Furthermore, it broke parity with middle cleaning (which includes inlined cross-crate re-exports) re-regressing parts of #44306 I once fixed in PR #103885.
**(1)** Lastly, PR #139035 introduced an ICE triggered by the following input file:
```rs
trait Trait { fn anon(()) {} } // internal error: entered unreachable code
```
---
This PR fixes all of these regressions and in the first commit renames several types and fns to be more ~~correct~~ descriptive and legible.
~~It also refactors `Param.name` to be of type `Option<Symbol>` instead `Symbol` (where `None` ~ `kw::Empty`), so rendering mistakes like that can no longer creep in like that (ignoring tests). CC #137978.~~ Independently done in PR #139846 a day prior.
cbf26629c4