Add FCW for derive helper attributes that will conflict with built-in attributes

[nik-rev]

Adds a future-compatibility-warning deny-by-default lint that helps catch invalid derive helper attribute names early.

This issues the lint, saying that ignore helper will clash with the built-in ignore attribute.

#![crate_type = "proc-macro"]
#![deny(ambiguous_derive_helpers)] 
use proc_macro::TokenStream; 

#[proc_macro_derive(Trait, attributes(ignore))]
pub fn example(input: TokenStream) -> TokenStream {
    TokenStream::new()
}

If you actually tried to use that ignore helper attribute, you won't be able to due to the ambiguity:

#[derive(Trait)]
struct Foo {
    #[ignore]
    field: (),
}

Produces:

error[E0659]: `ignore` is ambiguous
 --> src/lib.rs:5:7
  |
5 |     #[ignore]
  |       ^^^^^^ ambiguous name
  |
  = note: ambiguous because of a name conflict with a builtin attribute
  = note: `ignore` could refer to a built-in attribute
note: `ignore` could also refer to the derive helper attribute defined here
 --> src/lib.rs:3:10
  |
3 | #[derive(Trait)]
  |          ^^^^^