Use allocate_optimized_out_value instead of set_value_optimized_out.

Allocate the value as optimized out from the start rather than allocating
a value with contents, and then marking it optimized out.

gdb/
2013-07-04  Pedro Alves  <palves@redhat.com>

	* findvar.c (value_of_register): Use allocate_optimized_out_value
	if the register has been optimized out, instead of
	set_value_optimized_out.
	* frame-unwind.c (frame_unwind_got_optimized): Use
	allocate_optimized_out_value.