Move dwarf_call to dwarf_expr_context

Move dwarf_call to dwarf_expr_context

Following the idea of merging the evaluators, the dwarf_call and
get_frame_pc method can be moved from dwarf_expr_executor and
dwarf_evaluate_loc_desc classes to their base class dwarf_expr_context.
Once this is done, the get_frame_pc can be replace with lambda
function.

gdb/ChangeLog:

	* dwarf2/expr.c (dwarf_expr_context::dwarf_call): Move from
	dwarf_evaluate_loc_desc.
	(dwarf_expr_context::get_frame_pc): Replace with lambda.
	* dwarf2/expr.h (dwarf_expr_context::get_frame_pc): Remove
	method.
	* dwarf2/frame.c (dwarf_expr_executor::dwarf_call): Remove
	method.
	(dwarf_expr_executor::get_frame_pc): Remove method.
	* dwarf2/loc.c (dwarf_evaluate_loc_desc::get_frame_pc): Remove
	method.
	(dwarf_evaluate_loc_desc::dwarf_call): Move to
	dwarf_expr_context.
	(per_cu_dwarf_call): Inline function.