Poor diagnostics for signed/unsigned match

[jimblandy]

This class of error may go away entirely once we have implemented the abstract number types. But just in case:

fn f(u: u32) {
   switch (u) {
     case 1: { return; }
     default: { return; }
   }
}

produces:

error: expected unsigned/signed integer literal, found '1'
  ┌─ switch.wgsl:3:11
  │
3 │      case 1: { return; }
  │           ^ expected unsigned/signed integer literal

Could not parse WGSL

1 is definitely an "unsigned/signed integer literal".

Changing it to case 1u: causes validation to succeed.

[teoxoy]

closing as duplicate of #1996