rustc: Leak the LLVM module and context. #3552
auto merge of #7659 : brson/rust/leak-llvm, r=cmr

During normal batch operation, rustc will not bother to dispose of the LLVM module and context in order to save some time.

```
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
```