I first enabled AS deduction for references that allowed to inherit the right conversion diagnostics based on qualification conversion rules implemented earlier for the pointer type.
Then in order to tests the deduction rules fully, I had to enable some extra features from OpenCL 2.0 that are also valid in C++.
A number of ICEs fired in the CodeGen due to missing addrspacecast. Not convinced the current solution is good though. May be it would be cleaner to add a separate CastKind here - CK_LValueAddressSpaceConversion? Although I am not entirely clear about the benefits yet.