[clang][dataflow] Remove deprecated ValueModel::merge() function. (#82602)

I'm not aware of any remaining overrides of this function.

While I'm here, change an outdated comment in DataflowAnalysis.h that
still
referred to `merge()`. I've made the comment more general, referring
simply to
`ValueModel`, as we shouldn't really be repeating the documentation of
that
class here anyway.

Author: martinboehme

clang/include/clang/Analysis/FlowSensitive/DataflowAnalysis.h

@@ -54,10 +54,9 @@ namespace dataflow {
 ///                         Environment &Env)` - applies the analysis transfer
 ///    function for a given edge from a CFG block of a conditional statement.
 ///
-///  `Derived` can optionally override the following members:
-///   * `bool merge(QualType, const Value &, const Value &, Value &,
-///     Environment &)` -  joins distinct values. This could be a strict
-///     lattice join or a more general widening operation.
+///  `Derived` can optionally override the virtual functions in the
+///  `Environment::ValueModel` interface (which is an indirect base class of
+///  this class).
 ///
 ///  `LatticeT` is a bounded join-semilattice that is used by `Derived` and must
 ///  provide the following public members:

clang/include/clang/Analysis/FlowSensitive/DataflowEnvironment.h

@@ -79,32 +79,6 @@ class Environment {
       return ComparisonResult::Unknown;
     }
 
-    /// DEPRECATED. Override `join` and/or `widen`, instead.
-    ///
-    /// Modifies `MergedVal` to approximate both `Val1` and `Val2`. This could
-    /// be a strict lattice join or a more general widening operation.
-    ///
-    /// If this function returns true, `MergedVal` will be assigned to a storage
-    /// location of type `Type` in `MergedEnv`.
-    ///
-    /// `Env1` and `Env2` can be used to query child values and path condition
-    /// implications of `Val1` and `Val2` respectively.
-    ///
-    /// Requirements:
-    ///
-    ///  `Val1` and `Val2` must be distinct.
-    ///
-    ///  `Val1`, `Val2`, and `MergedVal` must model values of type `Type`.
-    ///
-    ///  `Val1` and `Val2` must be assigned to the same storage location in
-    ///  `Env1` and `Env2` respectively.
-    virtual bool merge(QualType Type, const Value &Val1,
-                       const Environment &Env1, const Value &Val2,
-                       const Environment &Env2, Value &MergedVal,
-                       Environment &MergedEnv) {
-      return true;
-    }
-
     /// Modifies `JoinedVal` to approximate both `Val1` and `Val2`. This should
     /// obey the properties of a lattice join.
     ///
@@ -121,11 +95,7 @@ class Environment {
     ///  `Env1` and `Env2` respectively.
     virtual void join(QualType Type, const Value &Val1, const Environment &Env1,
                       const Value &Val2, const Environment &Env2,
-                      Value &JoinedVal, Environment &JoinedEnv) {
-      [[maybe_unused]] bool ShouldKeep =
-          merge(Type, Val1, Env1, Val2, Env2, JoinedVal, JoinedEnv);
-      assert(ShouldKeep && "dropping merged value is unsupported");
-    }
+                      Value &JoinedVal, Environment &JoinedEnv) {}
 
     /// This function may widen the current value -- replace it with an
     /// approximation that can reach a fixed point more quickly than iterated