Allow llvm::Optional to work with types without default constructors.

This generalizes Optional to require less from the T type by using aligned
storage for backing & placement new/deleting the T into it when necessary.

Also includes unit tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175580 91177308-0d34-0410-b5e6-96231b3b80d8
Fix #includes, so we include only what we really need.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175581 91177308-0d34-0410-b5e6-96231b3b80d8