Swift: Add QL skeleton for aliased types.

Author: MathiasVP

swift/ql/lib/codeql/swift/elements/type/Type.qll

@@ -2,4 +2,13 @@ private import codeql.swift.generated.type.Type
 
 class Type extends Generated::Type {
   override string toString() { result = this.getName() }
+
+  /**
+   * Gets this type after any type aliases have been resolved. For example in
+   * the following code, the underlying type of `MyInt` is `Int`:
+   * ```
+   * typealias MyInt = Int
+   * ```
+   */
+  Type getUnderlyingType() { result = this }
 }

swift/ql/lib/codeql/swift/elements/type/TypeAliasType.qll

@@ -1,4 +1,16 @@
-// generated by codegen/codegen.py, remove this comment if you wish to edit this file
+private import codeql.swift.elements.type.Type
 private import codeql.swift.generated.type.TypeAliasType
 
-class TypeAliasType extends Generated::TypeAliasType { }
+class TypeAliasType extends Generated::TypeAliasType {
+  /**
+   * Gets the aliased type of this type alias type.
+   *
+   * For example the aliased type of `MyInt` in the following code is `Int`:
+   * ```
+   * typealias MyInt = Int
+   * ```
+   */
+  Type getAliasedType() { none() } // TODO: not yet implemented.
+
+  override Type getUnderlyingType() { result = this } // TODO: not yet implemented.
+}