gdb: add type::is_declared_class / type::set_is_declared_class

gdb: add type::is_declared_class / type::set_is_declared_class

Add the `is_declared_class` and `set_is_declared_class` methods on
`struct type`, in order to remove the `TYPE_DECLARED_CLASS` macro.  In
this patch, the macro is changed to use the getter, so all the call
sites of the macro that are used as a setter are changed to use the
setter method directly.  The next patch will remove the macro
completely.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <is_declared_class,
	set_is_declared_class>: New methods.
	(TYPE_DECLARED_CLASS): Use type::is_declared_class, change all
	write call sites to use type::set_is_declared_class.

Change-Id: Idf08d32e137c885a0aba0a18f556a899c1cbfd68