[Sema] Fix how we set implicit conversion kinds.

We have invariants we like to guarantee for the
`ImplicitConversionKind`s in a `StandardConversionSequence`. These
weren't being upheld in code that r280553 touched, so Richard suggested
that we should fix that. See D24113.

I'm not entirely sure how to go about testing this, so no test case is
included. Suggestions welcome.

llvm-svn: 280562