coverage: Skip instrumenting a function if no spans were extracted from MIR
The immediate symptoms of #118643 were fixed by #118666, but some users reported that their builds now encounter another coverage-related ICE:
```
error: internal compiler error: compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs:98:17: A used function should have had coverage mapping data but did not: (...)
```
I was able to reproduce at least one cause of this error: if no relevant spans could be extracted from a function, but the function contains `CoverageKind::SpanMarker` statements, then codegen still thinks the function is instrumented and complains about the fact that it has no coverage spans.
This PR prevents that from happening in two ways:
- If we didn't extract any relevant spans from MIR, skip instrumenting the entire function and don't create a `FunctionCoverateInfo` for it.
- If coverage codegen sees a `CoverageKind::SpanMarker` statement, skip it early and avoid creating `func_coverage`.
---
Fixes#118850.
The tests in this directory are shared by two different test modes, and can be
run in multiple different ways:
./x.py test coverage-map (compiles to LLVM IR and checks coverage mappings)
./x.py test coverage-run (runs a test binary and checks its coverage report)
./x.py test coverage (runs both coverage-map and coverage-run)
Maintenance note
These tests can be sensitive to small changes in MIR spans or MIR control flow,
especially in HIR-to-MIR lowering or MIR optimizations.
If you haven't touched the coverage code directly, and the tests still pass in
coverage-run mode, then it should usually be OK to just re-bless the mappings
as necessary with ./x.py test coverage-map --bless, without worrying too much
about the exact changes.
