[llvm] [AMDGPU] Re-enable closed-world assumption as an opt-out feature (PR #115371)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> What format are they provided in? IR or binary? If they are binary, it is essentially calling from one device image to another. If they are IR, we should have seen them at this moment.

They're external symbols provided by the linker. Think of it like this.
```c
extern int *foo_begin;
extern int *foo_end;
```
```c
static int foo[] = {1, 3, 4};
int *foo_begin = foo;
int *foo_end = foo + sizeof(foo);
```
```console
ld.lld foo_use.o foo_def.o
```
Except that it's all internal to the linker.

https://github.com/llvm/llvm-project/pull/115371