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

[Jessica Paquette via cfe-commits]

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@@ -941,6 +941,19 @@ struct CounterCoverageMappingBuilder
     return Counter::getCounter(CounterMap[S]);
   }
 
+  std::pair<Counter, Counter> getBranchCounterPair(const Stmt *S,
+                                                   Counter ParentCnt) {
+    Counter ExecCnt = getRegionCounter(S);
+    return {ExecCnt, Builder.subtract(ParentCnt, ExecCnt)};
+  }
+
+  bool IsCounterEqual(Counter OutCount, Counter ParentCount) {
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ornata wrote:

Should OutCount and ParentCount be const?

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