[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;
+    Counter Skipped;
+  };
+
+  BranchCounterPair getBranchCounterPair(const Stmt *S, Counter ParentCnt) {
+    Counter ExecCnt = getRegionCounter(S);
+    return {ExecCnt, Builder.subtract(ParentCnt, ExecCnt)};
+  }
+
+  /// Returns {TrueCnt,FalseCnt} for "implicit default".
+  /// FalseCnt is considered as the False count on SwitchStmt.
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chapuni wrote:

`getSwitchImplicitDefaultCounterPair` has been migrated to #113112. I suppose this explained what it did.
Note, I didn't migrate this using `BranchCounterPair` since I thought this was semantically different from others.

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