Slightly extend help diagnostic for immediately invoked closure

[Rageking8]

Given the following code: link

fn main() {
    println!("{}", || 123 ());
}

The current output is:

Compiling playground v0.0.1 (/playground)
error[[E0618]](https://doc.rust-lang.org/nightly/error-index.html#E0618): expected function, found `{integer}`
 --> src/main.rs:2:23
  |
2 |     println!("{}", || 123 ());
  |                       ^^^---
  |                       |
  |                       call expression requires function

For more information about this error, try `rustc --explain E0618`.
error: could not compile `playground` due to previous error

Ideally the output should suggest adding parentheses around the closure before calling it, since that suggestion is already present if the return value of the closure is wrapped in braces, as shown below.

fn main() {
    println!("{}", || { 123 } ());
}

Output with help suggestion:

Compiling playground v0.0.1 (/playground)
error[[E0618]](https://doc.rust-lang.org/nightly/error-index.html#E0618): expected function, found `{integer}`
 --> src/main.rs:2:23
  |
2 |     println!("{}", || { 123 } ());
  |                       ^^^^^^^---
  |                       |
  |                       call expression requires function
  |
help: if you meant to create this closure and immediately call it, surround the closure with parentheses
  |
2 |     println!("{}", (|| { 123 }) ());
  |                    +          +

For more information about this error, try `rustc --explain E0618`.
error: could not compile `playground` due to previous error

[compiler-errors]

This issue might benefit from a better title, maybe talking about the parsing associativity of () and closure exprs.