[NFC][mlir] Update DataFlowFramework.h to be compatible with clang c++23 (#152040)

This change makes `DataFlowFramework.h` compatible with `clang++` and
`--std=c++23`.

Previously clang was checking the templated `DataFlowSolver::eraseState`
body before being instantiated. This resulted in issues with incomplete
types, and happened at least with `clang++-19`.

This is fixed by moving the definition of `DataFlowSolver::eraseState`
after the `AnalysisState`'s full class declaration.

For full context:
-
https://discourse.llvm.org/t/what-is-the-status-of-c-23-support-in-mlir/87674/12

Author: philass

mlir/include/mlir/Analysis/DataFlowFramework.h

@@ -354,29 +354,7 @@ class DataFlowSolver {
 
   /// Erase any analysis state associated with the given lattice anchor.
   template <typename AnchorT>
-  void eraseState(AnchorT anchor) {
-    LatticeAnchor latticeAnchor(anchor);
-
-    // Update equivalentAnchorMap.
-    for (auto &&[TypeId, eqClass] : equivalentAnchorMap) {
-      if (!eqClass.contains(latticeAnchor)) {
-        continue;
-      }
-      llvm::EquivalenceClasses<LatticeAnchor>::member_iterator leaderIt =
-          eqClass.findLeader(latticeAnchor);
-
-      // Update analysis states with new leader if needed.
-      if (*leaderIt == latticeAnchor && ++leaderIt != eqClass.member_end()) {
-        analysisStates[*leaderIt][TypeId] =
-            std::move(analysisStates[latticeAnchor][TypeId]);
-      }
-
-      eqClass.erase(latticeAnchor);
-    }
-
-    // Update analysis states.
-    analysisStates.erase(latticeAnchor);
-  }
+  void eraseState(AnchorT anchor);
 
   /// Erase all analysis states.
   void eraseAllStates() {
@@ -560,6 +538,36 @@ class AnalysisState {
   friend class DataFlowSolver;
 };
 
+//===----------------------------------------------------------------------===//
+// DataFlowSolver definition
+//===----------------------------------------------------------------------===//
+// This method is defined outside `DataFlowSolver` and after `AnalysisState`
+// to prevent issues around `AnalysisState` being used before it is defined.
+template <typename AnchorT>
+void DataFlowSolver::eraseState(AnchorT anchor) {
+  LatticeAnchor latticeAnchor(anchor);
+
+  // Update equivalentAnchorMap.
+  for (auto &&[TypeId, eqClass] : equivalentAnchorMap) {
+    if (!eqClass.contains(latticeAnchor)) {
+      continue;
+    }
+    llvm::EquivalenceClasses<LatticeAnchor>::member_iterator leaderIt =
+        eqClass.findLeader(latticeAnchor);
+
+    // Update analysis states with new leader if needed.
+    if (*leaderIt == latticeAnchor && ++leaderIt != eqClass.member_end()) {
+      analysisStates[*leaderIt][TypeId] =
+          std::move(analysisStates[latticeAnchor][TypeId]);
+    }
+
+    eqClass.erase(latticeAnchor);
+  }
+
+  // Update analysis states.
+  analysisStates.erase(latticeAnchor);
+}
+
 //===----------------------------------------------------------------------===//
 // DataFlowAnalysis
 //===----------------------------------------------------------------------===//