rustc_span: mark few methods as must_use

[klensy]

Marked methods sometimes use returned value, sometimes not. I'm not sure that this is incorrect, so currently silenced warnings.

[rustbot]

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[jieyouxu]

This seems okay to me, but I'd like @petrochenkov to double-check in case one of the ignores actually shouldn't have been

[jieyouxu]

r? @petrochenkov

[petrochenkov]

I don't think must_use makes sense on any of those.
They are typical &mut self methods that are used for mutation, but can also return something useful in the process.