Remove the parent signature from consideration when computing the
generic signature for an extension. This cuts off a series of crashers
that involved nested extensions with trailing where clauses a la
extension Foo {
extension Foo where Self.Undefined == Bar {
}
}
The inner (invalid) extension technically has a parent signature from
Foo itself. Adding that signature to the GSB means when we go to
register the inner extension's generic parameters we crash.
Since extensions have to occur at the top level, just remove the parent
signature from consideration.
Fixes rdar://55502661
