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

[Jessica Paquette via cfe-commits]

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@@ -2221,27 +2249,27 @@ struct CounterCoverageMappingBuilder
     extendRegion(E->getRHS());
     propagateCounts(getRegionCounter(E), E->getRHS());
 
+    if (llvm::EnableSingleByteCoverage)
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ornata wrote:

I think that in the future, it would be really useful to add debug output to this code. It is becoming quite complex, with all of the types of coverage...

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