Support cfg attributes on function arguments

[floze-the-genius]

Summary

• preserve #[cfg] attributes when loading function arguments into the IR
• emit cfg-aware C/C++ parameter lists with valid comma placement
• emit void when every conditional argument is disabled
• add regression fixtures for conditional arguments in leading, middle, trailing, and all-conditional positions

Fixes #1037.

Testing

• CBINDGEN_TEST_VERIFY=1 CBINDGEN_TEST_NO_COMPILE=1 CARGO_BUILD_JOBS=8 cargo test test_cfg_function_args
• compiled generated C and C++ expectations with neither define, each define independently, and both defines
• cargo fmt -- --check
• CARGO_BUILD_JOBS=8 cargo clippy --workspace -- -D warnings
• CARGO_BUILD_JOBS=8 cargo check --all-targets --all-features
• CARGO_BUILD_JOBS=8 cargo package
• stable integration run: 153/159 fixtures passed; the remaining six macro-expansion fixtures require nightly because stable rejects -Zunpretty=expanded, matching the repository CI split

AI disclosure

I used OpenAI Codex to assist with issue discovery, implementation, and test execution. I reviewed and understand the changes and verified them locally.