[OpenCL] Fix initialisation of this via pointer.
When the expression used to initialise 'this' has a pointer type,
check the address space of the pointee type instead of the pointer
type to decide whether an address space cast is required.
It is the pointee type that carries the address space qualifier.
Patch by kpet (Kevin Petit)!
Differential Revision: https://reviews.llvm.org/D61319