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

[Jessica Paquette via cfe-commits]

================
@@ -941,6 +941,19 @@ struct CounterCoverageMappingBuilder
     return Counter::getCounter(CounterMap[S]);
   }
 
+  std::pair<Counter, Counter> getBranchCounterPair(const Stmt *S,
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ornata wrote:

it's not immediately obvious what the pair represents from this function name.

`getExecutionCountAnd...`?

or, make a lightweight POD struct to represent this

```
struct ExecCntAnd... { 
  Counter ExecCnt
  Counter ...
};
```

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