Allow closure arguments types to unify even if we can't fully resolve

a trait obligation. Partial fix for #16440 -- closure return types are
not handled yet.

src/test/run-pass/closure-inference.rs

@@ -14,6 +14,6 @@ fn foo(i: int) -> int { i + 1 }
 fn apply<A, F>(f: F, v: A) -> A where F: FnOnce(A) -> A { f(v) }
 
 pub fn main() {
-    let f = {|: i| foo(i)};
+    let f = {|i| foo(i)};
     assert_eq!(apply(f, 2), 3);
 }