fix issue printing the forget statement in the AST

`kw_forget` isn't covered by `operator<<` since it's
not a normal keyword.

Author: kavon

lib/AST/ASTPrinter.cpp

@@ -5177,7 +5177,7 @@ void PrintAST::visitThrowStmt(ThrowStmt *stmt) {
 }
 
 void PrintAST::visitForgetStmt(ForgetStmt *stmt) {
-  Printer << tok::kw_forget << " ";
+  Printer << "_forget" << " ";
   visit(stmt->getSubExpr());
 }
 

test/expr/print/forget.swift

@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend -enable-experimental-move-only -print-ast %s 2>&1 | %FileCheck %s
+
+@_moveOnly
+struct S {
+  __consuming func c() {
+    _forget self
+  }
+}
+
+// CHECK: @_moveOnly internal struct S {
+// CHECK:   internal __consuming func c() {
+// CHECK:     _forget self
+// CHECK:   }