[ASTImporter] Improved import of AlignedAttr.

Summary:
It is not enough to clone the attributes at import.
They can contain reference to objects that should be imported.
This work is done now for AlignedAttr.

Reviewers: martong, a.sidorin, shafik

Reviewed By: shafik

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, teemperor, martong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75048