Rollup merge of #118852 - Zalathar:no-spans, r=cjgillot

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.

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -85,6 +85,14 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
 
         let bx = self;
 
+        match coverage.kind {
+            // Marker statements have no effect during codegen,
+            // so return early and don't create `func_coverage`.
+            CoverageKind::SpanMarker => return,
+            // Match exhaustively to ensure that newly-added kinds are classified correctly.
+            CoverageKind::CounterIncrement { .. } | CoverageKind::ExpressionUsed { .. } => {}
+        }
+
         let Some(function_coverage_info) =
             bx.tcx.instance_mir(instance.def).function_coverage_info.as_deref()
         else {
@@ -100,9 +108,9 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
 
         let Coverage { kind } = coverage;
         match *kind {
-            // Span markers are only meaningful during MIR instrumentation,
-            // and have no effect during codegen.
-            CoverageKind::SpanMarker => {}
+            CoverageKind::SpanMarker => unreachable!(
+                "unexpected marker statement {kind:?} should have caused an early return"
+            ),
             CoverageKind::CounterIncrement { id } => {
                 func_coverage.mark_counter_id_seen(id);
                 // We need to explicitly drop the `RefMut` before calling into `instrprof_increment`,