Change attr_form_is_block to be a method

Change attr_form_is_block to be a method

This changes attr_form_is_block to be a method.  This is done
separately because, unlike the other attribute functions,
attr_form_is_block had special handling for the case where the
argument was NULL.  This required auditing each call site; in most
cases, NULL was already ruled out, but in a few spots, an additional
check was needed.

gdb/ChangeLog
2020-02-08  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (read_call_site_scope)
	(handle_data_member_location, dwarf2_add_member_fn)
	(mark_common_block_symbol_computed, read_common_block)
	(attr_to_dynamic_prop, partial_die_info::read)
	(var_decode_location, dwarf2_fetch_die_loc_sect_off)
	(dwarf2_symbol_mark_computed, set_die_type): Update.
	* dwarf2/attribute.h (struct attribute) <form_is_block>: Declare
	method.
	(attr_form_is_block): Don't declare.
	* dwarf2/attribute.c (attribute::form_is_block): Now a method.

Change-Id: Idfb290c61d738301ab991666f43e0b9cf577b2ae