Forward-declare raw_ostream (NFC)

This patch adds forward declarations of raw_ostream to those header
files that are relying on the forward declaration of raw_ostream in
llvm/include/llvm/ADT/Optional.h.

I'm planning to move operator<< for Optional<T> and std::optional<T>
from Optional.h to llvm/include/llvm/Support/raw_ostream.h.  Once I do
so, we no longer need to forward-declare raw_ostream in Optional.h.