[clang] [Coverage] Introduce `getBranchCounterPair()`. NFC. (PR #112702)

[Jessica Paquette via cfe-commits]

================
@@ -938,6 +938,37 @@ struct CounterCoverageMappingBuilder
     return Counter::getCounter(CounterMap[S]);
   }
 
+  struct BranchCounterPair {
+    Counter Executed;
----------------
ornata wrote:

Comments on member variables?

E.g.

```
/// Counter tracking number of times the branch was encountered and taken.
Counter Executed;

/// Counter tracking the number of times the branch was encountered, but not taken.
Counter Skipped;
```

https://github.com/llvm/llvm-project/pull/112702