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

[Jessica Paquette via cfe-commits]

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@@ -1709,6 +1730,13 @@ struct CounterCoverageMappingBuilder
             : addCounters(
                   addCounters(ParentCount, BackedgeCount, BodyBC.ContinueCount),
                   IncrementBC.ContinueCount);
+    auto [ExecCount, ExitCount] =
+        (llvm::EnableSingleByteCoverage
+             ? std::make_pair(getRegionCounter(S), Counter::getZero())
+             : getBranchCounterPair(S, CondCount));
+    if (!llvm::EnableSingleByteCoverage) {
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ornata wrote:

wrap in `#ifndef NDEBUG`?

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