clashing_extern_decl lint does not follow naming conventions

[nagisa]

@petrochenkov said here:

The lint doesn't follow the lint naming conventions - https://github.com/rust-lang/rfcs/blob/master/text/0344-conventions-galore.md#lints.

It should preferably be renamed clashing_extern_decl -> clashing_extern_declarations until it's on stable.

cc @jumbatm

This issue has been assigned to @jumbatm via this comment.

[jumbatm]

@rustbot claim