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

[NAKAMURA Takumi via cfe-commits]

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@@ -1592,6 +1605,13 @@ struct CounterCoverageMappingBuilder
         llvm::EnableSingleByteCoverage
             ? getRegionCounter(S->getCond())
             : addCounters(ParentCount, BackedgeCount, BC.ContinueCount);
+    auto [ExecCount, ExitCount] =
+        (llvm::EnableSingleByteCoverage
+             ? std::make_pair(getRegionCounter(S), Counter::getZero())
+             : getBranchCounterPair(S, CondCount));
+    if (!llvm::EnableSingleByteCoverage) {
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chapuni wrote:

Do you want me to apply the big request like #113115 w/o splitting?
I think it is required for step-by-step changes.

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