[ASTImporter] Corrected import of repeated friend declarations.

Summary:
Import declarations in correct order if a class contains
multiple redundant friend (type or decl) declarations.
If the order is incorrect this could cause false structural
equivalences and wrong declaration chains after import.

Reviewers: a.sidorin, shafik, a_sidorin

Reviewed By: shafik

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75740