Class-fy partial_die_info

Class-fy partial_die_info

This patch is to class-fy partial_die_info.  Two things special here,

 - disable assignment operator, but keep copy ctor, which is used in
   load_partial_dies,
 - have a private ctor which is only used by dwarf2_cu::find_partial_die,
   I don't want other code use it, so make it private,

gdb:

2018-02-26  Yao Qi  <yao.qi@linaro.org>

	* dwarf2read.c (struct partial_die_info): Add ctor, delete
	assignment operator.
	(load_partial_dies): Use ctor and copy ctor.
	(read_partial_die): Update.
	(dwarf2_cu::find_partial_die): Use ctor.