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

[NAKAMURA Takumi via cfe-commits]

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@@ -938,6 +938,37 @@ struct CounterCoverageMappingBuilder
     return Counter::getCounter(CounterMap[S]);
   }
 
+  struct BranchCounterPair {
+    Counter Executed;
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chapuni wrote:

I still wonder which pair of names would fit best. I adopted "Exec" from `BinOp` stuff and applied "Skipped" since "Exit" would be an exaggeration. "Checked/Skipped" would make less sense. OTOH, "Taken/NotTaken" looks redundant. Better idea?

I want them self-explained.

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