Add attribute::as_boolean method

This adds a new attribute::as_boolean method, and updates the reader
to use it.  The main benefit of this change is that now the code will
respect the attribute's form.

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

	* dwarf2/read.c (read_func_scope, prototyped_function_p)
	(read_subroutine_type, partial_die_info::read)
	(dwarf2_flag_true_p, new_symbol, dump_die_shallow)
	(dwarf2_add_member_fn): Update.
	* dwarf2/attribute.h (struct attribute) <as_boolean>: Declare.
	* dwarf2/attribute.c (attribute::as_boolean): New method.