[analyzer] Do not attempt to get the pointee of void*

Do not attempt to get the pointee of void* while generating a bug report 
(otherwise it will trigger an assert inside RegionStoreManager::getBinding 
assert(!T->isVoidType() && "Attempting to dereference a void pointer!")).

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D42396

llvm-svn: 323382