Detect type ascription use that should be let assignment

[estebank]

The following code

    let mut foo: i32 = 1;
    foo: i32 = 4;

currently emits

error[E0658]: type ascription is experimental
 --> src/main.rs:3:5
  |
3 |     foo: i32 = 4;
  |     ^^^^^^^^
  |
  = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
  = help: add `#![feature(type_ascription)]` to the crate attributes to enable

but it should also suggest let foo: i32 = 4;