[ADT, Support] Move operator<< to raw_ostream.h (NFC)

Without this patch, operator<< for Optional<T> and std::optional<T>
are in Optional.h.  This means that a C++ source file must include
Optional.h even if it just needs to stream std::optional<T> and has
nothing to do with Optional<T>, which is counter-intuitive.

This patch moves the operator<< to raw_ostream.h.

As a bonus, we get to resolve a circular dependency.  Optional.h no
longer needs to forward-declare raw_ostream.  That is, raw_ostream.h
depends on Optional.h, not vice versa.

As a preparation for this patch, I've checked in
77609717 to forward-declare
raw_ostream in those header files that were relying on the forward
declaration of raw_ostream in Optional.h.

Differential Revision: https://reviews.llvm.org/D139290