fix for find references skipping fields set in struct literals that happen to be after a return statement

[xraxra]

I noticed sometimes find references wouldn't list things, it turns out if I'm using a struct literal and it's placed below a return statement in the file, it will be skipped

its due to the position_context.returns becoming stale as it walks over the file (since returns is checked before assign)

simple repro scenario:

package main

Config :: struct {
	width: int,
}

// when there is a proc with return anywhere earlier in the file,
// find references on a struct field is unable to see places where that field is set inside a struct literal
unrelated :: proc() {
	return
}

apply :: proc() {
	config: Config
	config = { 	// the struct/compound literal
		width = 1, // find references on 'width' will miss this line
	}
	config.width = 1 // but this one is found
}

[BradLewis]

Thanks!