C#: Cleaup the QL implementation.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/exprs/Call.qll

@@ -267,6 +267,25 @@ class Call extends Expr, @call {
 class MethodCall extends Call, QualifiableExpr, LateBindableExpr, @method_invocation_expr {
   override Method getTarget() { expr_call(this, result) }
 
+  /**
+   * Gets the accessor that was used to generate this method, if any. For example, the
+   * method call `MyExtensions.get_FirstChar(s)` on line 9 is generated from the property
+   * accessor `get_FirstChar` on line 3 in
+   * ```csharp
+   * static class MyExtensions {
+   *   extension(string s) {
+   *     public char FirstChar { get { ... } }
+   *   }
+   * }
+   *
+   * class A {
+   *   char M(string s) {
+   *     return MyExtensions.get_FirstChar(s);
+   *   }
+   * }
+   */
+  Accessor getTargetAccessor() { expr_call(this, result) }
+
   override Method getQualifiedDeclaration() { result = this.getTarget() }
 
   override string toString() { result = "call to method " + concat(this.getTarget().getName()) }
@@ -600,8 +619,8 @@ class FunctionPointerCall extends DelegateLikeCall, @function_pointer_invocation
 
 /**
  * A call to an accessor. Either a property accessor call (`PropertyCall`),
- * an indexer accessor call (`IndexerCall`), an event accessor call
- * (`EventCall`) or an extension accessor call (`ExtensionAccessorCall`).
+ * an indexer accessor call (`IndexerCall`) or an event accessor call
+ * (`EventCall`).
  */
 class AccessorCall extends Call, QualifiableExpr, @call_access_expr {
   override Accessor getTarget() { none() }
@@ -611,35 +630,6 @@ class AccessorCall extends Call, QualifiableExpr, @call_access_expr {
   override Accessor getARuntimeTarget() { result = Call.super.getARuntimeTarget() }
 }
 
-/**
- * A direct call to an extension accessor, for example `MyExtensions.get_FirstChar(s)`
- * in
- * ```csharp
- * static class MyExtensions {
- *   extension(string s) {
- *     public char FirstChar { get { ... } }
- *   }
- * }
- *
- * class A {
- *   char M(string s) {
- *     return MyExtensions.get_FirstChar(s);
- *   }
- * }
- * ```
- */
-class ExtensionAccessorCall extends AccessorCall, @accessor_invocation_expr {
-  override Accessor getTarget() { expr_call(this, result) }
-
-  override Expr getArgument(int i) { result = this.getChild(i) and i >= 0 }
-
-  override string toString() {
-    result = "call to extension accessor " + concat(this.getTarget().getName())
-  }
-
-  override string getAPrimaryQlClass() { result = "ExtensionAccessorCall" }
-}
-
 /**
  * A call to a property accessor, for example the call to `get_P` on
  * line 5 in