coverage: Make BcbCounter module-private

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -153,12 +153,8 @@ fn create_mappings<'tcx>(
         &source_file.name.for_scope(tcx.sess, RemapPathScopeComponents::MACRO).to_string_lossy(),
     );
 
-    let term_for_bcb = |bcb| {
-        coverage_counters
-            .bcb_counter(bcb)
-            .expect("all BCBs with spans were given counters")
-            .as_term()
-    };
+    let term_for_bcb =
+        |bcb| coverage_counters.term_for_bcb(bcb).expect("all BCBs with spans were given counters");
     let region_for_span = |span: Span| make_source_region(source_map, file_name, span, body_span);
 
     // Fully destructure the mappings struct to make sure we don't miss any kinds.