Make some dwarf_expr_context methods pure virtual

Make some dwarf_expr_context methods pure virtual

This patch changes some dwarf_expr_context to be pure virtual, as
mentioned during the discussion of an earlier patch in this series.

2016-10-21  Tom Tromey  <tom@tromey.com>

	* dwarf2expr.h (class dwarf_expr_context)
	<get_frame_base, get_frame_cfa, get_tls_address, dwarf_call,
	push_dwarf_block_entry_value, get_addr_index, get_object_address>:
	Now pure virtual.
	* dwarf2-frame.c (class dwarf_expr_executor)
	<get_frame_base, get_frame_cfa, get_tls_address, dwarf_call,
	push_dwarf_block_entry_value, get_addr_index, get_object_address>:
	New methods.
	<invalid>: New method.