Fix AnnotatedType#stripTypeVar to not strip the annotation

[smarter]

This fixes Types#hasAnnotation which calls stripTypeVar which means that
it always returned false before.

@olhotak : please review

[olhotak]

Determining the correct fix to this is the subject of issue #325. That issue has only questions and no answers.

A problem is that much of the code throughout dotty relies on the current behaviour: there are many instances of tpe.stripTypeVar match { ... }, where the match cases do not handle typevars or annotated types. Fixing that was the intent of PR #350, but that was a huge invasive change, so it was dropped.

Probably the best compromise is to merge this change now, and fix call sites of stripTypeVar that rely on it to also strip annotations individually and iteratively when they are triggered by specific tests. So, LGTM.

[smarter]

Equivalent fix merged as part of #394.