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

[NAKAMURA Takumi via cfe-commits]

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

`BranchCounterPair` is a local type and only `getBranchCounterPair()` emits it for now. I think it'd be enough just to annotate members inside.

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