c++: implicit 'this' in noexcept-spec within class tmpl [PR94944]

c++: implicit 'this' in noexcept-spec within class tmpl [PR94944]

Here when instantiating the noexcept-spec we fail to resolve the
implicit object for the member call A<T>::f() ultimately because
maybe_instantiate_noexcept sets current_class_ptr/ref to the dependent
'this' (of type B<T>) rather than the specialized 'this' (of type B<int>).

This patch fixes this by making maybe_instantiate_noexcept set
current_class_ptr/ref to the specialized 'this' instead, consistent
with what tsubst_function_type does when substituting into the trailing
return type of a non-static member function.

	PR c++/94944

gcc/cp/ChangeLog:

	* pt.cc (maybe_instantiate_noexcept): For non-static member
	functions, set current_class_ptr/ref to the specialized 'this'
	instead.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept34.C: Adjusted expected diagnostics.
	* g++.dg/cpp0x/noexcept75.C: New test.