[clang] [clang][dataflow] Refactor `widen` API to be explicit about change effect. (PR #87233)

[Yitzhak Mandelbaum via cfe-commits]

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@@ -805,6 +805,25 @@ class NullPointerAnalysis final
     else
       JoinedVal.setProperty("is_null", JoinedEnv.makeTopBoolValue());
   }
+
+  std::optional<WidenResult> widen(QualType Type, Value &Prev,
+                                   const Environment &PrevEnv, Value &Current,
+                                   Environment &CurrentEnv) override {
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ymand wrote:

I added this so that we can test the widening functionality. Turns out we had no tests for framework calls to `widen`.

https://github.com/llvm/llvm-project/pull/87233