c++: constrained nested partial specialization [PR92103]

c++: constrained nested partial specialization [PR92103]

When determining the most specialized partial specialization of a
primary template that is nested inside a class template, we first
tsubst the outer template arguments into the TEMPLATE_DECL of each
partial specialization, and then check for satisfaction of each of the
new TEMPLATE_DECL's constraints.

But tsubst_template_decl does not currently guarantee that constraints
from the original DECL_TEMPLATE_RESULT get reattached to the new
DECL_TEMPLATE_RESULT.  In the testcase below, this leads to the
constraints_satisfied_p check in most_specialized_partial_spec to
trivially return true for each of the partial specializations.

I'm not sure if such a guarantee would be desirable, but in this case we
can just check constraints_satisfied_p on the original TEMPLATE_DECL
instead of on the tsubsted TEMPLATE_DECL here, which is what this patch
does (alongside some reorganizing).

gcc/cp/ChangeLog:

	PR c++/92103
	* pt.c (most_specialized_partial_spec): Reorganize the loop over
	DECL_TEMPLATE_SPECIALIZATIONS.  Check constraints_satisfied_p on
	the original template declaration, not on the tsubsted one.

gcc/testsuite/ChangeLog:

	PR c++/92103
	* g++.dg/cpp2a/concepts-partial-spec7.C: New test.