lint: don't suggest MaybeUninit::assume_init for uninhabited types

[y21]

Creating a zeroed uninhabited type such as ! or an empty enum with mem::zeroed() (or transmuting () to !) currently triggers this lint:

warning: the type `!` does not permit zero-initialization
 --> test.rs:5:23
  |
5 |         let _val: ! = mem::zeroed();
  |                       ^^^^^^^^^^^^^
  |                       |
  |                       this code causes undefined behavior when executed
  |                       help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
  |
  = note: the `!` type has no valid value

The MaybeUninit suggestion in the help message seems confusing/useless for uninhabited types, as such a type cannot be fully initialized in the first place (as the note implies).
This PR limits this help message to inhabited types which can be initialized

[rustbot]

r? @jackh726

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[jackh726]

r=me with rebase

[bors]

☔ The latest upstream changes (presumably #108211) made this pull request unmergeable. Please resolve the merge conflicts.

[y21]

Rebased and squashed the commits into one

[y21]

@jackh726, is there anything else I need to do here? Just making sure so it doesn't get lost or run into merge conflicts again.

[jackh726]

Nope

@bors r+

[bors]

📌 Commit 0610df9 has been approved by jackh726

It is now in the queue for this repository.