c++: Fix deferred noexcept on constructor [PR93901].

c++: Fix deferred noexcept on constructor [PR93901].

My change in r10-4394 to only update clones when we actually instantiate a
deferred noexcept-spec broke this because deferred parsing updates the
primary function but not the clones.  For GCC 10 I just reverted that
change; this patch adjusts maybe_instantiate_noexcept to update only the
clone passed as the argument.

gcc/cp/ChangeLog
2020-05-14  Jason Merrill  <jason@redhat.com>

	PR c++/93901
	* pt.c (maybe_instantiate_noexcept): Change clone handling.