Add DesugaringKind::FormatLiteral

[mejrs]

Implements DesugaringKind::FormatLiteral to mark the FormatArgs desugaring of format literals. The main use for this is to stop yapping about about formatting parameters if we're not anywhere near a format literal. The other use case is to fix suggestions such as #141350. It might also be useful for new or existing diagnostics that check whether they're in a format-like macro.

cc @xizheyin @fmease

[rustbot]

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[rustbot]

Some changes occurred in compiler/rustc_ast_lowering/src/format.rs

cc @m-ou-se

[fmease]

We might want to block this on #140748, I'm relatively sure. Just FYI

[mejrs]

Why is that?

[fmease]

Idk, if it's not conflicting according to git and if it's not stepping on each other's toes in other ways, I guess it's fine — I only skimmed boths diffs. Mara's PR will likely get approved soon anyway.

[xizheyin]

Great, #141633 can be refactored after this.

[SparrowLii]

This is very meaningful and the impl looks fine. But adding a new desugaringKind for diagnostics needs some opinions IMO
r? diagnostics