Allow named return values to be uninitialized

[Tetralux]

Currently, you are not allowed to do this:

Result :: struct {
  value: int,
}
f :: proc() -> (result: Result = ---) {
  result.value = 44
  return
}

This currently produces the following error:

Default parameter must be a constant, 2

Since named return values are literally variables that you can take the address of, etc, it makes sense to be to leave them uninitialized, in a similar manner to normal local variables, like in #1514.

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