c++: ICE with deferred noexcept when deducing targs [PR82099]

c++: ICE with deferred noexcept when deducing targs [PR82099]

In this test we ICE in type_throw_all_p because it got a deferred
noexcept which it shouldn't.  Here's the story:

In noexcept61.C, we call bar, so we perform overload resolution.  When
adding the (only) candidate, we need to deduce template arguments, so
call fn_type_unification as usually.  That deduces U to

  void (*) (int &, int &)

which is correct, but its noexcept-spec is deferred_noexcept.  Then
we call add_function_candidate (bar), wherein we try to create an
implicit conversion sequence for every argument.  Since baz<int> is
of unknown type, we instantiate_type it; it is a TEMPLATE_ID_EXPR
so that calls resolve_address_of_overloaded_function.  But we crash
there, because target_type contains the deferred_noexcept.

So we need to maybe_instantiate_noexcept before we can compare types.
resolve_overloaded_unification seemed like the appropriate spot, now
fn_type_unification produces the function type with its noexcept-spec
instantiated.  This shouldn't go against CWG 1330 because here we
really need to instantiate the noexcept-spec.

This also fixes class-deduction76.C, a dg-ice test I recently added,
therefore this fix also fixes c++/90799, yay.

gcc/cp/ChangeLog:

	PR c++/82099
	* pt.c (resolve_overloaded_unification): Call
	maybe_instantiate_noexcept after instantiating the function
	decl.

gcc/testsuite/ChangeLog:

	PR c++/82099
	* g++.dg/cpp1z/class-deduction76.C: Remove dg-ice.
	* g++.dg/cpp0x/noexcept61.C: New test.