Rollup merge of #103478 - SpanishPear:spanishpear/issue_103366_fix, r=TaKO8Ki

 Suggest fix for misplaced generic params on fn item #103366

fixes #103366

This still has some work to go, but works for 2/3 of the initial base cases described in #1033366

simple fn:
```
error: expected identifier, found `<`
 --> shreys/test_1.rs:1:3
  |
1 | fn<T> id(x: T) -> T { x }
  |   ^ expected identifier
  |
help: help: place the generic parameter list after the function name:
  |
1 | fn id<T>(x: T) -> T { x }
  |    ~~~~

```

Complicated bounds
```
error: expected identifier, found `<`
 --> spanishpear/test_2.rs:1:3
  |
1 | fn<'a, B: 'a + std::ops::Add<Output = u32>> f(_x: B) { }
  |   ^ expected identifier
  |
help: help: place the generic parameter list after the function name:
  |
1 | fn f<'a, B: 'a + std::ops::Add<Output = u32>>(_x: B) { }
  |    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Opening a draft PR for comments on approach, particularly I have the following questions:
 -  [x]  Is it okay to be using `err.span_suggestion` over struct derives? I struggled to get the initial implementation (particularly the correct suggestion message) on struct derives, although I think given what I've learned since starting, I could attempt re-doing it with that approach.
  -  [x] in the case where the snippet cannot be obtained from a span, is the `help` but no suggestion okay? I think yes (also, when does this case occur?)
  -  [x] are there any red flags for the generalisation of this work for relevant item kinds (i.e. `struct`, `enum`, `trait`, and `union`). My basic testing indicates it does work for those types except the help tip is currently hardcoded to `after the function name` - which should change dependent on the item.
  - [x] I am planning to not show the suggestion if there is already a `<` after the item identifier, (i.e. if there are already generics, as after a function name per the original issue). Any major objections?
  - [x] Is the style of error okay? I wasn't sure if there was a way to make it display nicer, or if thats handled by span_suggestion

These aren't blocking questions, and I will keep working on:
  - check if there is a `<` after the ident (and if so, not showing the suggestion)
  - generalize the help message
  - figuring out how to write/run/etc ui tests (including reading the docs for them)
  - logic cleanups

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