c++: alias-tmpl equivalence and default targs [PR103852]

c++: alias-tmpl equivalence and default targs [PR103852]

The suggested resolution for CWG1286, which we implemented, ignores default
template arguments, but this PR is an example of why that doesn't make
sense: the templates aren't functionally equivalent.

	PR c++/103852
	DR 1286

gcc/cp/ChangeLog:

	* pt.cc (get_underlying_template): Compare default template args.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-dr1286a.C: Default args now matter.
	* g++.dg/cpp1z/class-deduction-alias1.C: New test.