[clang] [clang][dataflow] Add synthetic fields to `RecordStorageLocation` (PR #73860)

[via cfe-commits]

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@@ -92,11 +96,39 @@ class DataflowAnalysisContext {
                               /*Logger=*/nullptr});
   ~DataflowAnalysisContext();
 
+  /// Sets a callback that returns the names and types of the synthetic fields
+  /// to add to a `RecordStorageLocation` of a given type.
+  /// Typically, this is called from the constructor of a `DataflowAnalysis`
+  ///
+  /// To maintain the invariant that all `RecordStorageLocation`s of a given
+  /// type have the same fields:
+  /// *  The callback must always return the same result for a given type
+  /// *  `setSyntheticFieldCallback()` must be called before any
+  //     `RecordStorageLocation`s are created.
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martinboehme wrote:

> Can this be integrated into the constructor? Could be in a followup patch (to minimize further churn in this one).

Unfortunately, that doesn't work because `setSyntheticFieldCallback()` needs to be called by the `DataflowAnalysis`, and the `DataflowAnalysis` doesn't create the `DataflowAnalysisContext` (i.e. it can't pass anything to its constructor).

I think we could consider changing this though: I don't think there's anything that would prevent the `DataflowAnalysis` itself from creating the `DataflowAnalysisContext`, and then the synthetic field callback could simply be a constructor parameter. This would require changing existing analyses, however, so I don't think this is something to tackle as part of this patch.

https://github.com/llvm/llvm-project/pull/73860