In order to support indirect vararg calls, we need to have information about the
callee type - this patch adds a `callee_type` attribute that holds that.
The attribute is required for vararg calls, else, it is optional and the callee
type is inferred by the operands and results of the operation if not present.
The syntax for non-vararg calls remains the same, whereas for vararg calls, it
is changed to this:
```
llvm.call %p(%arg0, %arg0) vararg(!llvm.func<void (i32, ...)>) : !llvm.ptr, (i32, i32) -> ()
llvm.call @s(%arg0, %arg0) vararg(!llvm.func<void (i32, ...)>) : (i32, i32) -> ()
```
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