[libc] Rework 'cpp:optional' to support move construction

This patch replaces the existing `cpp::optional` type with a newer
version that has more features. This class is heavily inspired by the
old `llvm::Optional` class. Currently the limitations of this class is
that we only handle types with trivial constructors or operators.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149972