Rust: avoid multiple PrimaryQLClass warnings

Fixes things like:
```
multiplePrimaryQlClasses
| x | PathExpr,VariableAccess |
```

rust/ql/lib/codeql/rust/elements/internal/VariableImpl.qll

@@ -1,6 +1,8 @@
 private import rust
 private import codeql.rust.elements.internal.generated.ParentChild
 private import codeql.rust.elements.internal.ExprImpl::Impl as ExprImpl
+private import codeql.rust.elements.internal.PathExprImpl::Impl as PathExprImpl
+private import codeql.rust.elements.internal.ImplicitVariableAccessImpl::Impl as ImplicitVariableAccessImpl
 private import codeql.util.DenseRank
 
 module Impl {
@@ -450,7 +452,8 @@ module Impl {
   private class TVariableAccess = Synth::TPathExpr or Synth::TImplicitVariableAccess;
 
   /** A variable access. */
-  class VariableAccess extends ExprImpl::Expr, TVariableAccess instanceof VariableAccessCand {
+  abstract class VariableAccess extends ExprImpl::Expr, TVariableAccess instanceof VariableAccessCand
+  {
     private string name;
     private Variable v;
 
@@ -467,6 +470,18 @@ module Impl {
     override string getAPrimaryQlClass() { result = "VariableAccess" }
   }
 
+  private class VariableAccessPathExpr extends VariableAccess, PathExprImpl::PathExpr {
+    override string getAPrimaryQlClass() { result = VariableAccess.super.getAPrimaryQlClass() }
+  }
+
+  private class VariableAccessImplicitVariableAccess extends VariableAccess,
+    ImplicitVariableAccessImpl::ImplicitVariableAccess
+  {
+    override string toString() { result = VariableAccess.super.toString() }
+
+    override string getAPrimaryQlClass() { result = VariableAccess.super.getAPrimaryQlClass() }
+  }
+
   /** Holds if `e` occurs in the LHS of an assignment or compound assignment. */
   private predicate assignmentExprDescendant(Expr e) {
     e = any(AssignmentExpr ae).getLhs()