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

[Jessica Paquette via cfe-commits]

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@@ -939,8 +939,17 @@ struct CounterCoverageMappingBuilder
     Counter Skipped;
   };
 
-  BranchCounterPair getBranchCounterPair(const Stmt *S, Counter ParentCnt) {
+  BranchCounterPair
+  getBranchCounterPair(const Stmt *S, Counter ParentCnt,
+                       std::optional<Counter> SkipCntForOld = std::nullopt) {
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ornata wrote:

Can you add a doxygen comment for `SkipCntForOld`?

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