[ADT] Make moving Optional not reset the Optional it moves from.

[ADT] Make moving Optional not reset the Optional it moves from.

This brings it in line with std::optional. My recent changes to
make Optional of trivial types trivially copyable introduced
diverging behavior depending on the type, which is bad. Now all
types have the same moving behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323445 91177308-0d34-0410-b5e6-96231b3b80d8