order to use the single assignment. That's probably worth doing for
a lot of these types anyways as they may have non-trivial moves and so
getting copy elision in more places seems worthwhile.

I've tried to add some tests that actually catch this mistake, and one
of the types is now well tested but the others' tests still fail to
catch this. I'll keep working on tests, but this gets the core pattern
right.

llvm-svn: 203780