rustc: Leak the LLVM module and context. #3552

```
before

time: 9.696 s   translation
time: 37.966 s  LLVM passes
time: 0.302 s   linking

real    1m4.415s
user    1m3.528s
sys     0m0.788s

after

time: 9.759 s   translation
time: 35.625 s  LLVM passes
time: 0.308 s   linking

real    1m3.528s
user    1m2.468s
sys     0m0.968s
```