A slightly more concise AST dump :)

If the modifiedType and the minimally desugared type of an AttributedType are the same, then we do not need to show both in an AST dump.

Here, we change VisitAttributedType to only visit the modifiedType when it is different from the equivalentType (the minimally desugared type), because the latter is already visited by default.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D142637