Add lval_funcs::is_optimized_out

This adds an is_optimized_out function pointer to lval_funcs, and
changes value_optimized_out to call it.  This new function lets gdb
determine if a value is optimized out without necessarily fetching the
value.  This is needed for a subsequent patch, where an attempt to
access a lazy value would fail due to the value size limit -- however,
the access was only needed to determine the optimized-out state.