c++: dump_template_bindings tweak

c++: dump_template_bindings tweak

in r12-1100 we stopped printing template bindings like T = T.  The check for
this relied on TREE_CHAIN of a TEMPLATE_TYPE_PARM holding the declaration of
that type-parameter.  This should be written as TYPE_STUB_DECL.  In
addition, TYPE_STUB_DECL is only set on the TYPE_MAIN_VARIANT, so we need to
check that as well.  Which is also desirable because volatile T is visibly
distinct from T.

gcc/cp/ChangeLog:

	* error.cc (dump_template_bindings): Correct skipping of
	redundant bindings.