c++: cast to array of unknown bound [PR93259]

We already know to treat a variable of array-of-unknown-bound type as
dependent, we should do the same for arr{}.

	PR c++/93259

gcc/cp/ChangeLog:

	* pt.cc (type_dependent_expression_p): Treat a compound
	literal of array-of-unknown-bound type like a variable.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/initlist-array17.C: New test.