read/write_pieced_value: Respect value parent's offset

read/write_pieced_value: Respect value parent's offset

In the case of targeting a bit-field, read_pieced_value and
write_pieced_value calculate the number of bits preceding the bit-field
without considering the relative offset of the value's parent.  This is
relevant for a structure variable like this:

  struct s {
      uint64_t foo;
      struct {
	  uint32_t bar;
	  uint32_t bf : 10;  /* <-- target bit-field */
      } baz;
  } s;

In this scenario, if 'val' is a GDB value representing s.baz.bf,
val->parent represents the whole s.baz structure, and the following holds:

  - value_offset (val) == sizeof s.baz.bar == 4
  - value_offset (val->parent) == sizeof s.foo == 8

The current logic would only use value_offset(val), resulting in the wrong
offset into the target value.  This is fixed.

gdb/ChangeLog:

	* dwarf2loc.c (read_pieced_value): Respect parent value's offset
	when targeting a bit-field.
	(write_pieced_value): Likewise.