Point at cause of expectation for type mismatch on `let` assignments with explicit type

[estebank]

Given

fn foo() -> i32 {
    let s: i32 = 3usize;
    s
}

we present

error[E0308]: mismatched types
 --> src/lib.rs:2:18
  |
2 |     let s: i32 = 3usize;
  |                  ^^^^^^ expected i32, found usize
help: change the type of the numeric literal from `usize` to `i32`
  |
2 |     let s: i32 = 3i32;
  |                  ^^^^

We would ideally also point at the assignment's type in the same way we do for type mismatches against a return type:

error[E0308]: mismatched types
 --> src/lib.rs:2:18
  |
2 |     let s: i32 = 3usize;
  |            ---   ^^^^^^ expected i32, found usize
  |            |
  |            expected due to this
help: change the type of the numeric literal from `usize` to `i32`
  |
2 |     let s: i32 = 3i32;
  |                  ^^^^