[clang][dataflow] Expose simple access to child StorageLocation presence. (llvm#145520)

`getChild` does not offer this knowledge, and a map lookup is
significantly cheaper than iteration over `children()`.

Author: bazuzi

clang/include/clang/Analysis/FlowSensitive/StorageLocation.h

@@ -168,6 +168,8 @@ class RecordStorageLocation final : public StorageLocation {
     return {Children.begin(), Children.end()};
   }
 
+  bool hasChild(const ValueDecl &D) const { return Children.contains(&D); }
+
 private:
   FieldToLoc Children;
   SyntheticFieldMap SyntheticFields;