This bug was present all along, but something changed in the order
of de-initialisation performed by llvm that makes it actually
crash now.
The constructor of HipifyPPCallbacks gives:
```
std::unique_ptr<HipifyPPCallbacks>(this)
```
to the LLVM Preprocessor instance. The Preprocessor instance
subsequently frees the HipifyPPCallbacks, which is then freed
again when we leave the stack frame at line 4340.
So: let's leak the HipifyPPCallbacks onto the heap, and leave
the LLVM Preprocessor object responsible for tidying it up.
[ROCm/clr commit: 893ee6d6ca]
Chris Kitching
d0feaee4f4