c++: correct __has_attribute(init_priority)

c++: correct __has_attribute(init_priority)

Currently __has_attribute(init_priority) always returns true, even on
targets that don't actually support init priorities, and when using the
attribute on such targets we just get a hard error about them being
unsupported.  This makes it impossible to conditionally use the attribute
by querying __has_attribute.

This patch fixes this by including init_priority in the attribute table
only if the target supports init priorities.  Thus on such targets
__has_attribute(init_priority) will now return false and we'll treat it
as just another unrecognized attribute (e.g. using it gives a -Wattribute
warning instead of a hard error).

gcc/cp/ChangeLog:

	* tree.cc (cxx_attribute_table): Include init_priority entry
	only if SUPPORTS_INIT_PRIORITY.
	(handle_init_priority_attribute): Add ATTRIBUTE_UNUSED.  Assert
	SUPPORTS_INIT_PRIORITY is true.

gcc/testsuite/ChangeLog:

	* g++.dg/special/initpri3.C: New test.