coverage-dump: Resolve global file IDs to filenames
The coverage-dump tool, used by coverage tests, currently includes “global file ID” numbers in its dump output.
This PR adds support for parsing coverage filename information from LLVM assembly `.ll` files, and resolving those file IDs to the corresponding filename, for inclusion in dump output.
This makes dump output more informative, especially for test cases involving multiple files, and will be important for testing expansion region support in the future.
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The bootstrap changes don't necessarily have to land at the same time (e.g. they could be deferred to after the stage0 redesign if requested), but I would prefer to land them now if possible.
42156bde4d