c++: unifying identical tmpls from current inst [PR108347]

Here more_specialized_partial_spec wrongly considers the two partial
specializations to be unordered ultimately because unify for identical
parm=arg=A<T>::C returns failure due to C being dependent.

This patch fixes this by relaxing unify's early-exit identity test to
also accept dependent decls; we can't deduce anything further from them
anyway.  In passing this patch removes the CONST_DECL case of unify:
we should never see the CONST_DECL version of a template parameter here,
and for other CONST_DECLs (such as enumerators) it seems we can rely on
them to already have been folded to their DECL_INITIAL.

	PR c++/108347

gcc/cp/ChangeLog:

	* pt.cc (unify): Return unify_success for identical dependent
	DECL_P 'arg' and 'parm'.
	<case CONST_DECL>: Remove handling.

gcc/testsuite/ChangeLog:

	* g++.dg/template/ttp41.C: New test.