[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
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ornata wrote:

if it's going to change further, then this is fine as-is

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