Rust: Refactor AstCfgNode to use a field

Co-authored-by: Tom Hvitved <[email protected]>

Author: paldepind

rust/ql/lib/codeql/rust/controlflow/CfgNodes.qll

@@ -8,13 +8,15 @@ private import ControlFlowGraph
 
 /** A CFG node that corresponds to an element in the AST. */
 class AstCfgNode extends CfgNode {
-  AstCfgNode() { exists(this.getAstNode()) }
+  AstNode node;
+  
+  AstCfgNode() { node = this.getAstNode() }
 }
 
 /** A CFG node that corresponds to an expression in the AST. */
 class ExprCfgNode extends AstCfgNode {
-  ExprCfgNode() { this.getAstNode() instanceof Expr }
+  override Expr node;
 
   /** Gets the underlying expression. */
-  Expr getExpr() { result = this.getAstNode() }
+  Expr getExpr() { result = node }
 }