[clang] [Coverage] Rework !SystemHeadersCoverage (PR #91446)

[Jessica Paquette via cfe-commits]

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@@ -2064,7 +2082,20 @@ struct CounterCoverageMappingBuilder
     createDecisionRegion(E, DecisionParams);
   }
 
+  /// Check if E belongs to system headers.
+  bool isExprInSystemHeader(const BinaryOperator *E) const {
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ornata wrote:

assert E is not nullptr?

https://github.com/llvm/llvm-project/pull/91446