[clang] [Clang] Allow all address spaces to be converted to the default (PR #112248)

[Alex Voicu via cfe-commits]

AlexVlx wrote:

> I think the default AS is expected to be losslessly converted to other AS but not the other way around, though I understand it is not clearly stated in LangRef or other places and a lot of code just assumes it can.

I don't think this expectation is thoroughly encoded anywhere, and it's definitely not binding today (there are targets who have a mapping of default that breaks this assumption). Whilst the premise is interesting I'm not sure this patch is doing the right thing. If you are in vanilla C/C++, you wouldn't have anything but default, and hopefully the default is mapped to something sane on the target. 

Running into an observable situation where this is a concern means that either you've messed around with (non C/C++) attributes, or are linking in something exciting. Neither of which constitutes valid C/C++, and I'm leaning towards saying shouldn't be allowed to silently work - we probably should refrain from infecting C/C++ with explicit address spaces a la OpenCL. What are you actually running into @jhuber6 where this is a concern?

https://github.com/llvm/llvm-project/pull/112248