[clang][dataflow] Output debug info if `getChild()` doesn't find field.

[mboehme]

Summary

Depends On D153409

[mboehme]

Pre-merge failures look unrelated

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

 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_ANALYSIS_FLOWSENSITIVE_STORAGELOCATION_H
 #define LLVM_CLANG_ANALYSIS_FLOWSENSITIVE_STORAGELOCATION_H
 
 #include "clang/AST/Decl.h"
 #include "clang/AST/Type.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/Support/Debug.h"
+
+#define DEBUG_TYPE "dataflow"
 
 namespace clang {
 namespace dataflow {
 
 /// Base class for elements of the local variable store and of the heap.
 ///
 /// Each storage location holds a value. The mapping from storage locations to
 /// values is stored in the environment.
public:
 
   static bool classof(const StorageLocation *Loc) {
     return Loc->getKind() == Kind::Aggregate;
   }
 
   /// Returns the child storage location for `D`.
   StorageLocation &getChild(const ValueDecl &D) const {
     auto It = Children.find(&D);
+    LLVM_DEBUG({
+      if (It == Children.end()) {
+        llvm::dbgs() << "Couldn't find child " << D.getNameAsString()
+                     << " on StorageLocation " << this << " of type "
+                     << getType() << "\n";
+        llvm::dbgs() << "Existing children:\n";
+        for ([[maybe_unused]] auto [Field, Loc] : Children) {
+          llvm::dbgs() << Field->getNameAsString() << "\n";
+        }
+      }
+    });
     assert(It != Children.end());
     return *It->second;
   }
 
   llvm::iterator_range<FieldToLoc::const_iterator> children() const {
     return {Children.begin(), Children.end()};
   }
 
 private:
   FieldToLoc Children;
 };
 
 } // namespace dataflow
 } // namespace clang
 
+#undef DEBUG_TYPE
+
 #endif // LLVM_CLANG_ANALYSIS_FLOWSENSITIVE_STORAGELOCATION_H