Arg splat experiment - syntax impl This PR is part of the argument splatting lang experiment, and FFI overloading / C++ interop project goals: - https://github.com/rust-lang/rust/issues/153629 - https://rust-lang.github.io/rust-project-goals/2026/overloading-for-ffi.html - https://rust-lang.github.io/rust-project-goals/2025h2/interop-problem-map.html I've split it from rust-lang/rust#153697 to make reviewing easier, see that PR for more details. The PR is the initial implementation of the feature: - `splat` incomplete feature gate - `#[splat]` attribute on function arguments - feature gate and UI tests for item type filtering, non-splattable arguments Once this PR merges, I'll rebase rust-lang/rust#153697.