Auto merge of #52089 - eddyb:issue-51907, r=nagisa

rustc_codegen_llvm: replace the first argument early in FnType::new_vtable.

Fixes #51907 by removing the vtable pointer before the `ArgType` is even created.
This allows any ABI to support trait object method calls, regardless of how it passes `*dyn Trait`.

r? @nikomatsakis

diff --cc src/librustc_codegen_llvm/abi.rs
Simple merge