[analyzer] Fix crash in GenericTaintChecker when propagatig taint to AllocaRegion

The `GenericTaintChecker` checker was crashing, when the taint
was propagated to `AllocaRegion` region in following code:
```
  int x;
  void* p = alloca(10);
  mempcy(p, &x, sizeof(x));
```
This crash was caused by the fact that determining type of
`AllocaRegion` returns a null `QualType`.

This patch makes `AllocaRegion` expose its type as `void`,
making them consistent with results of `malloc` or `new`
that produce `SymRegion` with `void*` symbol.

Reviewed By: steakhal, xazax.hun

Differential Revision: https://reviews.llvm.org/D155847