Add test suite `coverage-map` to test coverage mappings emitted by LLVM

We compile each test file to LLVM IR assembly, and then pass that IR to a
dedicated program that can decode LLVM coverage maps and print them in a more
human-readable format. We can then check that output against known-good
snapshots.

This test suite has some advantages over the existing `run-coverage` tests:

- We can test coverage instrumentation without needing to run target binaries.

- We can observe subtle improvements/regressions in the underlying coverage
mappings that don't make a visible difference to coverage reports.

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