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
• omit swift_name attributes when conditional parameters would make their fixed arity invalid
• add regression fixtures for conditional arguments in leading, middle, trailing, and all-conditional positions

Fixes #1037.

Testing

• CBINDGEN_TEST_VERIFY=1 cargo test --test tests with nightly on PATH: 159/159 passed
• generated C and C++ compiled with Clang using neither define, each define independently, and both defines
• cargo test --lib --bins: 4/4 passed
• cargo check --all-targets
• cargo fmt -- --check
• git diff --check
• cargo clippy --all-targets -- -D warnings reaches a pre-existing clippy::too_many_arguments finding in tests/tests.rs:31; the changed production code is not implicated

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.