c++: generic lambda and function ptr conv [PR105221]

c++: generic lambda and function ptr conv [PR105221]

We weren't properly considering the function pointer conversions in
deduction between FUNCTION_TYPE; we just hardcoded the
UNIFY_ALLOW_MORE_CV_QUAL semantics, which are backwards when deducing for a
template conversion function like the one in a generic lambda.  And when I
started checking the ALLOW flags, I needed to make sure they stay set to
avoid breaking trailing13.C.

	PR c++/105221

gcc/cp/ChangeLog:

	* pt.cc (unify) [FUNCTION_TYPE]: Handle function pointer
	conversions.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/noexcept-type27.C: New test.