Rollup merge of #121135 - Zalathar:no-whole-body-span, r=wesleywiser

coverage: Discard spans that fill the entire function body

While debugging some other coverage changes, I discovered a frustrating inconsistency that occurs in functions containing closures, if they end with an implicit `()` return instead of an explicit trailing-expression.

This turns out to have been caused by the corresponding node in MIR having a span that covers the entire function body. When preparing coverage spans, any span that fills the whole body tends to cause more harm than good, so this PR detects and discards those spans.

(This isn't the first time whole-body spans have caused problems; we also eliminated some of them in #118525.)

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