[OpenCL] Set generic addr space of 'this' in special class members.

Set address spaces of 'this' param correctly for implicit special
class members.

This also changes initialization conversion sequence to separate
address space conversion from other qualifiers in case of binding
reference to a temporary. In this case address space conversion  
should happen after the binding (unlike for other quals). This is
needed to materialize it correctly in the alloca address space.

Initial patch by Mikael Nilssoni!

Differential Revision: https://reviews.llvm.org/D56066

llvm-svn: 351053