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

[Jessica Paquette via cfe-commits]

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@@ -938,6 +938,37 @@ struct CounterCoverageMappingBuilder
     return Counter::getCounter(CounterMap[S]);
   }
 
+  struct BranchCounterPair {
+    Counter Executed;
+    Counter Skipped;
+  };
+
+  BranchCounterPair getBranchCounterPair(const Stmt *S, Counter ParentCnt) {
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ornata wrote:

Can we add Doxygen comments for this?

I'd like to be able to know what, e.g. `ParentCnt` is only by looking at this function.

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