Union types in type hints and static typehint

[i582]

RFC: https://wiki.php.net/rfc/union_types_v2

The static type hint can only be used on a union return type.
This must be taken into account in the grammar.

Examples:

function f(int|string $a) {}
function f(): int|string {}

class Foo {
	public int|string $a;
}