clear obligations-added flag with nested fulfillcx

This flag is a debugging measure designed to detect cases where we start
a snapshot, create type variables, register obligations involving those
type variables in the fulfillment cx, and then have to unroll the
snapshot, leaving "dangling type variables" behind.  HOWEVER, in some
cases the flag is wrong. In particular, we sometimes create a
"mini-fulfilment-cx" in which we enroll obligations. As long as this
fulfillment cx is fully drained before we return, this is not a problem,
as there won't be any escaping obligations in the main cx. So we add a
fn to save/restore the flag.

src/test/compile-fail/issue-36053-2.rs

@@ -0,0 +1,21 @@
+// Copyright 2016 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.
+
+// Regression test for #36053. ICE was caused due to obligations
+// being added to a special, dedicated fulfillment cx during
+// a probe.
+
+use std::iter::once;
+fn main() {
+    once::<&str>("str").fuse().filter(|a: &str| true).count();
+    //~^ ERROR no method named `count`
+    //~| ERROR E0281
+    //~| ERROR E0281
+}

src/test/run-pass/issue-36053.rs

@@ -0,0 +1,32 @@
+// Copyright 2016 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.
+
+// Regression test for #36053. ICE was caused due to obligations being
+// added to a special, dedicated fulfillment cx during a
+// probe. Problem seems to be related to the particular definition of
+// `FusedIterator` in std but I was not able to isolate that into an
+// external crate.
+
+#![feature(fused)]
+use std::iter::FusedIterator;
+
+struct Thing<'a>(&'a str);
+impl<'a> Iterator for Thing<'a> {
+    type Item = &'a str;
+    fn next(&mut self) -> Option<&'a str> {
+        None
+    }
+}
+
+impl<'a> FusedIterator for Thing<'a> {}
+
+fn main() {
+    Thing("test").fuse().filter(|_| true).count();
+}