Allow returning of temps and ro’s from lv subs

Allow returning of temps and ro’s from lv subs

This commit removes the restriction on returning temps and read-only
scalars from lvalue subs that occurs when the sub returns implicitly
(with no ‘return’ statement; ‘return’ has never had that restriction).

It does not actually help pure-Perl lvalue subs much yet, as op.c
still enforces lvalue syntax on the last statement.

But this should fix bug #71172, allowing XS lvalue subs to work under
the debugger.