SI-7853 A less ad-hoc place to call memberType

Rather than localizing the fix to the outerAccessor, this
commit pushed the call to `memberType` into *all* usages of
`newValDef` and `newDefDef`.

The TPT of `applyOrElse` in synthetized partial functions must
be set explicitly to pass the pos/t7853-partial-function.scala.
Otherwise, the as-seen-from ends up cloning the type parameter
`B1` of `applyOrElse` as it transforms (questionably)
its bound from `List[Int @unchecked]` to `List[Int]`.

Partial Function synthesis was already a delicate area, and this
makes things more explicit which could be counted as an improvement.