coverage: Don't store function_source_hash in BcbCounter::Counter

This shows one small benefit of separating `BcbCounter` from `CoverageKind`.
The function source hash will be the same for all counters within a function,
so instead of passing it through `CoverageCounters` and storing it in every
counter, we can just supply it during the final conversion to `CoverageKind`.

compiler/rustc_mir_transform/src/coverage/tests.rs

@@ -674,7 +674,7 @@ fn test_make_bcb_counters() {
                 ));
             }
         }
-        let mut coverage_counters = counters::CoverageCounters::new(0, &basic_coverage_blocks);
+        let mut coverage_counters = counters::CoverageCounters::new(&basic_coverage_blocks);
         coverage_counters
             .make_bcb_counters(&mut basic_coverage_blocks, &coverage_spans)
             .expect("should be Ok");