Remove DW_BLOCK

Remove DW_BLOCK

This removes the DW_BLOCK accessor in favor of methods on struct
attribute.  The methods, unlike the access, check the form.

Note that DW_FORM_data16 had to be handled by form_is_block, because
in practice that is how we store values of this form.

gdb/ChangeLog
2020-09-29  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (read_call_site_scope)
	(handle_data_member_location, dwarf2_add_member_fn)
	(mark_common_block_symbol_computed, attr_to_dynamic_prop)
	(partial_die_info::read, read_attribute_value)
	(var_decode_location, dwarf2_const_value_attr, dump_die_shallow)
	(dwarf2_fetch_die_loc_sect_off, dwarf2_fetch_constant_bytes)
	(dwarf2_symbol_mark_computed): Update.
	* dwarf2/attribute.h (struct attribute) <as_block, set_block>: New
	methods.
	(DW_BLOCK): Remove.
	* dwarf2/attribute.c (attribute::form_is_block): Add
	DW_FORM_data16.