OptionalStorage implementation for trivial type, take III

OptionalStorage implementation for trivial type, take III

This is another attempt at implementating optional storage
for trivially copyable type, using an union instead of a
raw buffer to hold the actual storage. This make it possible
to get rid of the reinterpret_cast, and hopefully to fix the UB
of the previous attempts.

This validates fine on my laptop for gcc 8.2 and gcc 4.8, I'll
revert if it breaks the validation.

llvm-svn: 354137