Use argument type directly from fflush if available in translation unit

When our RuntimeDebugBuilder calles fflush(NULL) to flush all output streams, it
is important that the types we use in the call match the ones used in a
declaration of fflush possible already available in the translation unit.

As we just pass on a NULL pointer, the type of the pointer value does not really
matter. However, as LLVM complains in case of mismatched types, we make sure
to create a NULL pointer of identical type.

No test case, as RuntimeDebugBuilder is not permanently used in Polly. Calls to
it are until now only used to add informative output during debugging sessions.