Make dwarf2_evaluate_property parameter const

dwarf2_evaluate_property should not modify its "addr_stack"
parameter's contents.  This patch makes this part of the API, by
marking it const.

gdb/ChangeLog
2020-03-20  Tom Tromey  <tromey@adacore.com>

	* dwarf2/loc.h (dwarf2_evaluate_property): Make "addr_stack"
	const.
	* dwarf2/loc.c (dwarf2_evaluate_property): Make "addr_stack"
	const.