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.

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -1500,6 +1500,18 @@ fn build_vtable_type_di_node<'ll, 'tcx>(
     .di_node
 }
 
+fn find_vtable_behind_cast<'ll>(vtable: &'ll Value) -> &'ll Value {
+    // The vtable is a global variable, which may be behind an addrspacecast.
+    unsafe {
+        if let Some(c) = llvm::LLVMIsAConstantExpr(vtable) {
+            if llvm::LLVMGetConstOpcode(c) == llvm::Opcode::AddrSpaceCast {
+                return llvm::LLVMGetOperand(c, 0).unwrap();
+            }
+        }
+    }
+    vtable
+}
+
 pub(crate) fn apply_vcall_visibility_metadata<'ll, 'tcx>(
     cx: &CodegenCx<'ll, 'tcx>,
     ty: Ty<'tcx>,
@@ -1520,6 +1532,7 @@ pub(crate) fn apply_vcall_visibility_metadata<'ll, 'tcx>(
 
     let Some(trait_ref) = trait_ref else { return };
 
+    let vtable = find_vtable_behind_cast(vtable);
     let trait_ref_self = trait_ref.with_self_ty(cx.tcx, ty);
     let trait_ref_self = cx.tcx.erase_regions(trait_ref_self);
     let trait_def_id = trait_ref_self.def_id();
@@ -1593,6 +1606,8 @@ pub(crate) fn create_vtable_di_node<'ll, 'tcx>(
         return;
     }
 
+    let vtable = find_vtable_behind_cast(vtable);
+
     // When full debuginfo is enabled, we want to try and prevent vtables from being
     // merged. Otherwise debuggers will have a hard time mapping from dyn pointer
     // to concrete type.