c++: Fix template arguments comparison with class NTTP [PR91754]

Here we fail to compile the attached test, stating that the use of
T<s> in T<s>::T() {} is "invalid use of incomplete type".  It is a
function definition so grokdeclarator checks that the qualifying type
is complete.

When we parsed the class T, finish_struct gave the class a non-null
TYPE_SIZE, making it COMPLETE_TYPE_P.  But then we're parsing T<s>,
a TEMPLATE_ID, in

  T<s>::T() {}

so try to lookup_template_class T.  This failed because we couldn't
find such a class: comp_template_args told us that the argument lists
don't match, because one of the args was wrapped in a VIEW_CONVERT_EXPR
to make it look const.  It seems to me that we should see through
these artificial wrappers and consider the args same.

2020-01-29  Marek Polacek  <polacek@redhat.com>

	PR c++/91754 - Fix template arguments comparison with class NTTP.
	* pt.c (class_nttp_const_wrapper_p): New.
	(template_args_equal): See through class_nttp_const_wrapper_p
	arguments.

	* g++.dg/cpp2a/nontype-class30.C: New test.

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