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

[NAKAMURA Takumi via cfe-commits]

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@@ -941,6 +941,19 @@ struct CounterCoverageMappingBuilder
     return Counter::getCounter(CounterMap[S]);
   }
 
+  std::pair<Counter, Counter> getBranchCounterPair(const Stmt *S,
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chapuni wrote:

Done but I am still dubious namings would be appropriate. Not all users expect `[Exec, Skip]`, but `[Exec, Exit]` or similar. Do you have a better idea to resolve namings (not by structural assignments but by name)?

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