Rollup merge of #139102 - Zalathar:no-split, r=oli-obk

coverage: Avoid splitting spans during span extraction/refinement

This PR removes or simplifies some of the steps involved in extracting coverage-relevant spans from MIR, and preparing them for use in coverage instrumentation metadata.

A common theme is that we now try harder to avoid modifying or combining spans in non-trivial ways, because those modifications present the most risk for weird behaviour or ICEs.

The main changes are:
- When extracting spans from MIR call terminators, try to restrict them to just the function name.
- Instead of splitting spans around “holes”, just discard any span that overlaps with a hole.
- Instead of splitting macro-invocation spans into two parts, truncate them to just the macro name and subsequent `!`.

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This results in a lot of tiny changes to the spans that end up in coverage metadata, and a few changes to coverage reports. Judging by test snapshots, these changes appear to be quite minor in practice.

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