[clang] [clang][CodeGen] Make `UnqualPtrTy` truly unqualified (PR #94388)

[Alex Voicu via cfe-commits]

AlexVlx wrote:

> I think the point is there are two things going on here that people think of as unqualified:
> 
> 1. "Give me a pointer that's whatever `void *` is"
> 2. "Give me a pointer in some address space just so I can put a global in some metadata"
> 
> The vast majority of the time we're talking about 1, which is what most people would think of unqualified as meaning. #93601 / #93914 are about the three special cases that are case 2 above. But those are a special case, and normally when people say unqualified they mean "default" address space / `void *`.

Building on the above, let me add a bit more colour I should have added from the get go:

  - More often than not, for case (1) above, folks probably want (knowingly or not) a `void*` that "works",  
    with no actual concern for the address  space (the "default");
  - As an example, for targets which model themselves / draw inspiration from e.g. OpenCL, this'd be a 
    generic / flat pointer, but there's no way for the FE to query that  from the target (today?), and that 
    information isn't really observably encoded in something like the DataLayout; if we were to add 
    something like that, then unqualified would / should correspond to this, IMHO (could well happen to be 0 
    in practice);
  - We have targets that map `LangAS::Default`oddly / differently from the above expectation, and hence it 
     appeared profitable to have `UnqualPtrTy`, which is a generally useful / everywhere usable pointer type, 
     be different from `Int8PtrTy` / `VoidPtrTy` which are pointer types which correspond to the target's 
     default (perhaps I am being overly cautious here though).

Having said that, this change is somewhat inert at the moment beyond carving out the name / making it shorthand for calling `getUnqual()`. In the future, we could either:

- extend DL to expose a target's flat/generic AS
- (more intrusive / might be infeasible) make unqualified `ptr`s be generic/everywhere usable in IR, without 
  the target exposing a flat/generic AS, letting the target figure out the legality of e.g. this or that ascast.

https://github.com/llvm/llvm-project/pull/94388