PR c++/87366 - wrong error with alias template.

PR c++/87366 - wrong error with alias template.

With this testcase the code in template_args_equal to treat aliases as
distinct wasn't sufficient, because it only looked at the top level, whereas
here we have a reference to the alias.  So let's also handle treating them
as distinct in structural_comptypes.  For GCC 10 I have a more comprehensive
patch, but for GCC 9 let's go with this smaller change.

	* typeck.c (structural_comptypes): When comparing_specializations,
	aliases are unequal.
	(comptypes): When comparing_specializations, do structural
	comparison.

From-SVN: r270494