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

[NAKAMURA Takumi via cfe-commits]

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

I assume each visitor (`visitBinLAnd` `visitBinLOr`) will not receive nullptr.

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