generic function mishandles unique pointer

(Based on test/run-pass/generic-drop-glue.rs)

```
fn f<T>(t: T) { let _t1 = t; }
fn main() { let x = ~1; f(x); }
```

Compiles to a program that double-frees (and leaks?):

```
g(10978,0xb0101000) malloc: *** error for object 0x1000000: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
rt: fatal, 'live_allocs < 1' failed, ../src/rt/memory_region.cpp:43 
```
