Allow DWARF expression to push the initial address

Allow DWARF expression to push the initial address

Some DWARF expressions must be evaluated by first pushing the object
address onto the evaluation stack.  This patch adds this ability.
This functionality is not used yet, but it will be used in a later
patch.  This is split out for easier review and also because it
improved the patch series ordering.

gdb/ChangeLog
2020-04-24  Tom Tromey  <tromey@adacore.com>

	* dwarf2/loc.c (dwarf2_locexpr_baton_eval): Add
	"push_initial_value" parameter.
	(dwarf2_evaluate_property): Likewise.
	* dwarf2/loc.h (dwarf2_evaluate_property): Update.