Move computed value's frame id to piece_closure

Move computed value's frame id to piece_closure

Nowadays, we set computed value's frame id, which is a misuse to me.
The computed value itself doesn't care about frame id, but function
value_computed_funcs (val)->read (or read_pieced_value) cares about
which frame the register is relative to, so 'struct piece_closure' is
a better place to fit frame id.

This patch adds a frame id in 'struct piece_closure', and use it
instead of using computed value's frame id.

gdb:

2016-11-28  Yao Qi  <yao.qi@linaro.org>

	* dwarf2loc.c (struct piece_closure) <frame_id>: New field.
	(allocate_piece_closure): Add new parameter 'frame' and set
	closure's frame_id field accordingly.
	(read_pieced_value): Get frame from closure instead of value.
	(dwarf2_evaluate_loc_desc_full): Remove code getting frame id.
	Don't set value's frame id.