Don't approximate a type using Nothing as prefix

[smarter]

Approximate range(Nothing, hi) to Nothing & hi instead of Nothing. This avoids
creating TypeRefs with a Nothing prefix which manifest themselves down the line
with an error like:

Cannot resolve reference to type path.type.AbsMember.
The classfile defining the type might be missing from the classpath.
     val z1 = transition(di).ext(1) // error
                        ^

Due to a MissingType thrown from TypeErasure#sigName.

This change required tweaking Type#findMember to handle WildcardTypes instead of
returning NoDenotation. This was only required for two
tests (tests/pos/i7414.scala and tests/pos/i13842.scala) where wildApprox ends
up creating a TermRef whose underlying type is a WildcardType, because of a type
parameter being substituted by a wildcard, which seems legitimate when we're using
wildApprox.

The presentation compiler also had a special case for Nothing appearing in
completion which had to be adapted.

Fixes #23530