[clang] [Clang] Add `-fdefault-generic-addrspace` flag for targeting GPUs (PR #115777)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > I agree in general that C/C++ has no semantic meaning ascribed to address spaces.
> 
> Err, C does. Please see TR 18037 (https://standards.iso.org/ittf/PubliclyAvailableStandards/c051126_ISO_IEC_TR_18037_2008.zip)
> 
> AIUI, we've extended the C extension into C++, so we should be following the C semantics while mapping them on to C++'s stronger type system as closely as we can.

Interesting, though reading through that I didn't see any mentions of implicit casts. It's simply stating that they can do casting if the target lists them as a subset. However, that would mean that the program passing sema would be dependent on the target machine. I'm actually not opposed to that, we could tear out the `isAddrSpaceSuperSetOf` check to allow it to keep a reference to the `TargetMachine` when making its decisions.

Also it's worth noting that in the documentation it has stuff like `__X char x, y, z` is a warning in `clang` due to how type level attributes work https://godbolt.org/z/Pfv36nEdq.

Maybe someone who reads better standardese than me can help with this. My ultimate goal is to make this legal when targeting NVPTX or AMDGPU, which is easily done by stating that `DefaultAS` is a superset of these other AS's. Let me know if making this target dependent is a good idea.
```c
void *foo(void [[clang::addrspace(3) *p) { return p; }
```

https://github.com/llvm/llvm-project/pull/115777