Manually fulfill lint expectations for all unsafe blocks with metavars

[y21]

Fixes #14488

Context: the macro_metavars_in_unsafe lint looks for unsafe blocks with a macro span that then contain expressions with a root context span (which means that it is a macro with an unsafe block expanding a metavariable inside). In order to avoid emitting a warning for every single macro invocation, it will deduplicate the unsafe blocks by the span in the macro.

This leads to the linked issue where because of the deduplicating and removing unsafe blocks that all belong to the same unsafe block in the macro, only one of the unsafe blocks will actually have its lint expectation fulfilled. This PR fixes that by manually fulfilling all of the unsafe blocks from all expansions before deduplicating them.

changelog: [macro_metavars_in_unsafe]: fix unfulfilled #[expect] if macro is invoked multiple times

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[samueltardieu]

Neat

[blyxyas]

Wouldn't this also affect to any lint that apply to macros? I'm not sure if there are more, but it may add more value to modify behaviour upstream?

For now, as I think there aren't any other lints affecting to macro_rules! declarations, we can accept this, but if other lints start to lint in macros, we may be interested in fixing this in the expectation lint behaviour handler in upstream.