Move push_dwarf_reg_entry_value to expr.c

Move push_dwarf_reg_entry_value to expr.c

Following the idea of merging the evaluators, the
push_dwarf_reg_entry_value method can be moved from
dwarf_expr_executor and dwarf_evaluate_loc_desc classes
to their base class dwarf_expr_context.

gdb/ChangeLog:

	* dwarf2/expr.c
        (dwarf_expr_context::push_dwarf_reg_entry_value): Move from
	dwarf_evaluate_loc_desc.
	* dwarf2/frame.c
	(dwarf_expr_executor::push_dwarf_reg_entry_value): Remove
	method.
	* dwarf2/loc.c (dwarf_expr_reg_to_entry_parameter): Expose
	function.
	(dwarf_evaluate_loc_desc::push_dwarf_reg_entry_value): Move to
	dwarf_expr_context.
	* dwarf2/loc.h (dwarf_expr_reg_to_entry_parameter): Expose
	function.