SI-8263 Avoid SOE in Symbol#logicallyEnclosingMember under Scala 2.11

Since the fix for SI-2066, Scala 2.11 calls logicallyEnclosingMember on the
`x` in the expansion of the task macro:

    InitializeInstance.app[[T0[x]](T0[java.io.File], T0[java.io.File]), Seq[java.io.File]]

This exposed the fact that SBT has created `T0` with `NoSymbol` as
the owner. This led to the a SOE.

I will also change the compiler to be more tolerant of this, but we
can observe good discipline in the macro and pick a sensible owner.