Change some uses of DW_STRING to string method

Change some uses of DW_STRING to string method

This changes some of the simpler spots to use attribute::string rather
than DW_STRING.

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

	* dwarf2/read.c (partial_die_info::read)
	(dwarf2_const_value_attr, anonymous_struct_prefix, )
	(dwarf2_name, dwarf2_fetch_constant_bytes): Use
	attribute::as_string.