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

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