ExistentialTypeTree.whereClauses are now MemberDefs

Today’s flight back to Lausanne wasn’t as productive as the recent flight
to Minsk (https://github.com/scala/scala/pull/3305), but I noticed
one minor thingie: ExistentialTypeTree had an imprecise type specified
for its whereClauses. This is now fixed.

I didn’t increment PickleFormat.*Version numbers, because this change
introduces only a miniscule incompatibility with what would have been
a meaningless and most likely crash-inducing pickle anyway.
Merge pull request #3332 from xeno-by/topic/existential-type-tree

ExistentialTypeTree.whereClauses are now MemberDefs