Auto merge of #116409 - Zalathar:span-extraction, r=oli-obk

coverage: Separate initial span extraction from span processing

One of the main subtasks of coverage instrumentation is looking through MIR to determine a list of source code spans that require coverage counters.

That task is in turn subdivided into a few main steps:
- Getting the initial spans from MIR statements/terminators
- Processing the list of spans to merge or truncate nearby spans as necessary
- Grouping the processed spans by their corresponding coverage graph node

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This PR enforces a firmer separation between the first two steps (span extraction and span processing), which ends up slightly simplifying both steps, since they don't need to deal with state that is only meaningful for the other step.

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`@rustbot` label +A-code-coverage