Store BCB counters externally, not directly in the BCB graph

Storing coverage counter information in `CoverageCounters` has a few advantages
over storing it directly inside BCB graph nodes:

- The graph doesn't need to be mutable when making the counters, making it
easier to see that the graph itself is not modified during this step.

- All of the counter data is clearly visible in one place.

- It becomes possible to use a representation that doesn't correspond 1:1 to
graph nodes, e.g. storing all the edge counters in a single hashmap instead of
several.

compiler/rustc_mir_transform/src/coverage/tests.rs

@@ -675,7 +675,7 @@ fn test_make_bcb_counters() {
                 ));
             }
         }
-        let mut coverage_counters = counters::CoverageCounters::new(0);
+        let mut coverage_counters = counters::CoverageCounters::new(0, &basic_coverage_blocks);
         let () = coverage_counters
             .make_bcb_counters(&mut basic_coverage_blocks, &coverage_spans)
             .expect("should be Ok");
@@ -684,7 +684,7 @@ fn test_make_bcb_counters() {
         let_bcb!(1);
         assert_eq!(
             0, // bcb1 has a `Counter` with id = 0
-            match basic_coverage_blocks[bcb1].counter().expect("should have a counter") {
+            match coverage_counters.bcb_counter(bcb1).expect("should have a counter") {
                 CoverageKind::Counter { id, .. } => id,
                 _ => panic!("expected a Counter"),
             }
@@ -694,7 +694,7 @@ fn test_make_bcb_counters() {
         let_bcb!(2);
         assert_eq!(
             1, // bcb2 has a `Counter` with id = 1
-            match basic_coverage_blocks[bcb2].counter().expect("should have a counter") {
+            match coverage_counters.bcb_counter(bcb2).expect("should have a counter") {
                 CoverageKind::Counter { id, .. } => id,
                 _ => panic!("expected a Counter"),
             }