Update what seems like an outdated comment

Totally minor, but I was taking a look at the source for educational purposes and noticed that it seems ASTNode evolved to be a union of a few additional types without the file comment being updated.   Updated to reflect the code, if I understand it correctly.

Author: haikusw

include/swift/AST/ASTNode.h

@@ -10,7 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This file defines the ASTNode, which is a union of Stmt, Expr, and Decl.
+// This file defines the ASTNode, which is a union of Stmt, Expr, Decl,
+// Pattern, TypeRepr, StmtCondition, and CaseLabelItem
 //
 //===----------------------------------------------------------------------===//