[clang] [AMDGPU] Enable overriding of OpenCL's default address space (PR #117588)

[Jakub Chlanda via cfe-commits]

jchlanda wrote:

> In principle I am against this, it adds a relatively brittle hook, and bypasses the pre-existing mechanisms (use CL2 or enable the generic-as extension) for obtaining this behaviour, in a way that does not ensure that the pre-existing mechanisms are available (e.g. it appears one could pass the option, without asking for the generic as extension on CL3.0).

That is fair, after having a second look additional chack for `Opts.OpenCLGenericAddressSpace` that [you have added](https://github.com/llvm/llvm-project/pull/112442) recently, is all we need to make it work. Closing it now.

https://github.com/llvm/llvm-project/pull/117588