[clang] [clang][dataflow] Add support for serialization and deserialization. (PR #152487)

[Yitzhak Mandelbaum via cfe-commits]

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@@ -157,10 +157,18 @@ class Environment {
   };
 
   /// Creates an environment that uses `DACtx` to store objects that encompass
-  /// the state of a program.
+  /// the state of a program. `FlowConditionToken` sets the flow condition
+  /// associated with the environment. Generally, new environments should be
+  /// initialized with a fresh token, by using one of the other
+  /// constructors. This constructor is for specialized use, including
+  /// deserialization and delegation from other constructors.
+  Environment(DataflowAnalysisContext &DACtx, Atom FlowConditionToken)
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ymand wrote:

The problem is that the exact serialization choice is up to clients -- there isn't one facility, like there is for Formula. The motivating example is Nullability inference, where we'll be serializing to custom protobufs. So, I don't think that C++'s encapsulation mechanism is a good fit here.

I'm open to alternative designs -- e.g. should we consider defining a protobuf message here, etc.

https://github.com/llvm/llvm-project/pull/152487