Rename _args -> args in format_args expansion

[dtolnay]

As observed in #91359 (comment), prior to that PR this variable was sometimes never used, such as in the case of:

println!("");

// used to expand to:
::std::io::_print(
    ::core::fmt::Arguments::new_v1(
        &["\n"],
        &match () {
            _args => [],
        },
    ),
);

so the leading underscore in _args was used to suppress an unused variable lint. However after #91359 the variable is always used when present, as the unused case would instead expand to:

::std::io::_print(::core::fmt::Arguments::new_v1(&["\n"], &[]));

[rust-highfive]

r? @cjgillot

(rust-highfive has picked a reviewer for you, use r? to override)

[cjgillot]

@bors r+ rollup

[bors]

📌 Commit 6667d78 has been approved by cjgillot