Support: Stop relying on iterator auto-conversion, NFC

Stop relying on ilist implicit conversions from `value_type&` to
`iterator` in YAMLParser.cpp.

I eventually want to outlaw this entirely.  It encourages
`getNextNode()` and `getPrevNode()` in iterator logic, which is
extremely fragile (and relies on them never returning `nullptr`).

FTR, there's nothing nefarious going on in this case, it was just easy
to clean up since the callers really wanted iterators to begin with.

llvm-svn: 249767