Rollup merge of #135026 - Flakebi:global-addrspace, r=saethlin

Cast global variables to default address space

Pointers for variables all need to be in the same address space for correct compilation. Therefore ensure that even if a global variable is created in a different address space, it is casted to the default address space before its value is used.

This is necessary for the amdgpu target and others where the default address space for global variables is not 0.

For example `core` does not compile in debug mode when not casting the address space to the default one because it tries to emit the following (simplified) LLVM IR, containing a type mismatch:

```llvm
`@alloc_0` = addrspace(1) constant <{ [6 x i8] }> <{ [6 x i8] c"bit.rs" }>, align 1
`@alloc_1` = addrspace(1) constant <{ ptr }> <{ ptr addrspace(1) `@alloc_0` }>, align 8
; ^ here a struct containing a `ptr` is needed, but it is created using a `ptr addrspace(1)`
```

For this to compile, we need to insert a constant `addrspacecast` before we use a global variable:

```llvm
`@alloc_0` = addrspace(1) constant <{ [6 x i8] }> <{ [6 x i8] c"bit.rs" }>, align 1
`@alloc_1` = addrspace(1) constant <{ ptr }> <{ ptr addrspacecast (ptr addrspace(1) `@alloc_0` to ptr) }>, align 8
```

As vtables are global variables as well, they are also created with an `addrspacecast`. In the SSA backend, after a vtable global is created, metadata is added to it. To add metadata, we need the non-casted global variable. Therefore we strip away an addrspacecast if there is one, to get the underlying global.

Tracking issue: #135024

Author: matthiaskrgr