Clear cached landing pads before generating a call.

Using the wrong landing pad has obvious bad effects, like dropping a value
twice.

Testcase written by Alex Crichton.

Fixes #25089.

src/test/run-pass/issue-25089.rs

@@ -0,0 +1,40 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::thread;
+
+struct Foo(i32);
+
+impl Drop for Foo {
+    fn drop(&mut self) {
+        static mut DROPPED: bool = false;
+        unsafe {
+            assert!(!DROPPED);
+            DROPPED = true;
+        }
+    }
+}
+
+struct Empty;
+
+fn empty() -> Empty { Empty }
+
+fn should_panic(_: Foo, _: Empty) {
+    panic!("test panic");
+}
+
+fn test() {
+    should_panic(Foo(1), empty());
+}
+
+fn main() {
+    let ret = thread::spawn(test).join();
+    assert!(ret.is_err());
+}