Make tail call expr mandatory

[dtolnay]

This was probably written just mirroring the grammar of return. But for become, the expression after the keyword is mandatory.

#![allow(incomplete_features)]
#![feature(explicit_tail_calls)]

fn repro() {
    become;
}

error: expected expression, found `;`
 --> src/lib.rs:5:11
  |
5 |     become;
  |           ^ expected expression