SI-8291 Fix implicitNotFound message with type aliases

This pattern of code is typically a bug:

    if (f(tp.typeSymbol)) {
       g(tp.typeArgs)
    }

Intead, one needs to take the base type of `tp` wrt `tp.typeSymbol`.

This commit does exactly that when formatting the `@implicitNotFound`
custom error message.

Patch found on the back of an envelope in the handwriting of @adriaanm
Merge pull request #4029 from retronym/ticket/8291

SI-8291 Fix implicitNotFound message with type aliases