Broken completion in display test cases

Happens because of `@:pos`.
And second bug, if you uncomment `// return fields;`, try completion in `Main.main`, it will work, but will stop after checking `Macro.hx` tab and moving back to `Main.hx`.
```haxe
@:build(Macro.buildTestCase())
class Main {
static function main() {
var foo = "foo";
}
}

Macro.hx:

import haxe.macro.Context;
import haxe.macro.Expr;

class Macro {
static function buildTestCase():Array {
var fields = Context.getBuildFields();
// return fields;
var c = Context.getLocalClass().get();
for (field in fields) {
switch (field.kind) {
case FFun(f) if (f.expr != null):
f.expr = macro @:pos(f.expr.pos) {
var abc = 0;
${f.expr}
};
case _:
}
}
return fields;
}
}
```