This reverts commit 8f0df9f3bb.
The Optional*RefDegradesTo*EntryPtr types want to keep the same size as
the underlying type, which std::optional doesn't guarantee. For use with
llvm::Optional, they define their own storage class, and there is no way
to do that in std::optional.
On top of that, that commit broke builds with older GCCs, where
std::optional was not trivially copyable (static_assert in the clang
sources was failing).
205c0589f9