swift234

Revert "ManagedValue forwardInto and assignInto should not always "take" their value."

This reverts commit r30272.

There's a way to fix Builtin.reinterpretCast that preserves SILGen
RValue invariants better according to JohnM. So although these changes
are valid, there will be no way for me to test them.

After reverting this, I will roll back in the change to
emitImplicitValueConstructor because I think the old code was
violating SILGen invariants.

Swift SVN r30306