coverage: Detect unused local file IDs to avoid an LLVM assertion

This case can't actually happen yet (other than via a testing flag), because
currently all of a function's spans must belong to the same file and expansion.
But this will be an important edge case when adding expansion region support.

compiler/rustc_interface/src/tests.rs

@@ -776,7 +776,8 @@ fn test_unstable_options_tracking_hash() {
         CoverageOptions {
             level: CoverageLevel::Mcdc,
             no_mir_spans: true,
-            discard_all_spans_in_codegen: true
+            discard_all_spans_in_codegen: true,
+            inject_unused_local_file: true,
         }
     );
     tracked!(crate_attr, vec!["abc".to_string()]);