source-gen: remove 'actual' for aliases in NT

For aliases, the transformX() method in NarrowingTransformer isn’t
actually 'actual', so AliasGenerator shouldn’t add that annotation in
this case.
Add MemberMeta

    `Iterable<String>.collect<Integer?>`

(For this, I had to turn Identifiers into ExpressionIshs. We need to use
LIdentifier instead of MemberNameWithTypeArguments for MetaQualifier
because type arguments aren’t allowed in a member qualifier.)