Fix fn_sig_for_fn_abi and the coroutine transform for generators

There were three issues previously:
* The self argument was pinned, despite Iterator::next taking an
  unpinned mutable reference.
* A resume argument was passed, despite Iterator::next not having one.
* The return value was CoroutineState<Item, ()> rather than Option<Item>

While these things just so happened to work with the LLVM backend,
cg_clif does much stricter checks when trying to assign a value to a
place. In addition it can't handle the mismatch between the amount of
arguments specified by the FnAbi and the FnSig.

rustfmt.toml

@@ -39,4 +39,5 @@ ignore = [
     # these are ignored by a standard cargo fmt run
     "compiler/rustc_codegen_cranelift/y.rs", # running rustfmt breaks this file
     "compiler/rustc_codegen_cranelift/scripts",
+    "compiler/rustc_codegen_cranelift/example/gen_block_iterate.rs", # uses edition 2024
 ]