SI-8263 Avoid SOE in logicallyEnclosingMember

We've started calling this method during higher-kinded subtyping
to ensure that variances of higher order type params in overriding
as soundly aligned.

Turns out that running this over the expansion of the SBT task
macro leads to a SOE due to a corrupt owner chain.

I've fixed that in SBT (https://github.com/sbt/sbt/pull/1113),
but we ought not crash like this.

This commit considers NoSymbol to be its own enclosing member and
logs a -Xdev warning. This is analagous to the handling of
`NoSymbol.owner`.