c++: __func__ and local class DMI [PR105809]

As in 108242, we need to instantiate in the context of the enclosing
function, not after it's gone.

	PR c++/105809

gcc/cp/ChangeLog:

	* init.cc (get_nsdmi): Split out...
	(maybe_instantiate_nsdmi_init): ...this function.
	* cp-tree.h: Declare it.
	* pt.cc (tsubst_expr): Use it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-__func__3.C: New test.