c++: dependence of member noexcept-spec [PR104079]

Here a stale TYPE_DEPENDENT_P/_P_VALID value for f's function type
after replacing the type's DEFERRED_NOEXCEPT with the parsed dependent
noexcept-spec causes us to try to instantiate g's noexcept-spec ahead
of time (since it in turn appears non-dependent), leading to an ICE.

This patch fixes this by clearing TYPE_DEPENDENT_P_VALID in
fixup_deferred_exception_variants appropriately (as in
build_cp_fntype_variant).

That turns out to fix the testcase for C++17 but not for C++11/14,
because it's not until C++17 that a noexcept-spec is part of (and
therefore affects dependence of) the function type.  Since dependence of
NOEXCEPT_EXPR is defined in terms of instantiation dependence, the most
appropriate fix for earlier dialects seems to be to make instantiation
dependence consider dependence of a noexcept-spec.

	PR c++/104079

gcc/cp/ChangeLog:

	* pt.cc (value_dependent_noexcept_spec_p): New predicate split
	out from ...
	(dependent_type_p_r): ... here.
	(instantiation_dependent_r): Use value_dependent_noexcept_spec_p
	to consider dependence of a noexcept-spec before C++17.
	* tree.cc (fixup_deferred_exception_variants): Clear
	TYPE_DEPENDENT_P_VALID.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept74.C: New test.
	* g++.dg/cpp0x/noexcept74a.C: New test.