c++: deducing only from noexcept-spec [PR80951]

c++: deducing only from noexcept-spec [PR80951]

The "fail-fast" predicate uses_deducible_template_parms used by
unify_one_argument is neglecting to look inside the noexcept-spec of a
function type.  This causes deduction to spuriously fail whenever the
noexcept-spec is the only part of the type which contains a deducible
template parameter.

	PR c++/80951

gcc/cp/ChangeLog:

	* pt.cc (uses_deducible_template_parms): Consider the
	noexcept-spec of a function type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/noexcept-type25.C: New test.