Update Clap to avoid future incompatibility warning

[matklad]

Ping from rust-lang/rust :-)

rustc has a bug in macro parsing for quite a while, where it didn't flag some erroneous macro definitions as such.

This was a deny-by-default lint for some time, and in https://github.com/rust-lang/rust/pull/75516 we want to turn it into a hard error.

Clap 2.9 is one of the affected crates.

This PR just updates clap to keep this crate compiling with Cargo.lock.