Avoid typing annotations twice

[smarter]

typedAnnotation was used both in Namer#addAnnotations and
Typer#completeAnnotations, resulting in some annotations being typed
twice. Fixed by introducing a typedAheadAnnotation (and renaming an
existing method with the same name that doesn't actually type the full
annotation tree).

[smarter]

@odersky Both Namer#addAnnotations and Typer#completeAnnotations define a lazy val annotCtx, but the definitions are different and don't always produce the same result, compare:
https://github.com/lampepfl/dotty/blob/78a5e092a834b76c54b18d5e58cb0a593f4bb8a5/compiler/src/dotty/tools/dotc/typer/Namer.scala#L741-L747
https://github.com/lampepfl/dotty/blob/78a5e092a834b76c54b18d5e58cb0a593f4bb8a5/compiler/src/dotty/tools/dotc/typer/Typer.scala#L1345-L1355

Is one "more correct" than the other? Can we come up with a single definition that we can use in both?

[smarter]

There's another issue which is that we're not actually using the annotCtx with Annotation.deferred:
https://github.com/lampepfl/dotty/blob/78a5e092a834b76c54b18d5e58cb0a593f4bb8a5/compiler/src/dotty/tools/dotc/typer/Namer.scala#L751
The ctx that will be passed here will just be the enclosing context.