[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 {
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ornata wrote:

IIUC the point of this pair is given:

```
if (x) {
  // ... do something
} else if (y) {
 // ... something else
} else {
 // ... another something else
}
```

You want to track, _from the perspective of a branch_, how many times it is executed.

For example, there will be a `BranchAndCounterPair` for the `if`, the `else if`, and the `else`.

Assuming I'm understanding this correctly, I think we could explain this struct like so:

```
/// Structure to track how many times a conditional was executed or skipped over when evaluated at runtime.
///
/// E.g. given a conditional like
///
/// if (x) {...}
/// 
/// Keep track of the number of times the `if` branch is taken, and the number of times it is not taken.
```

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