[ASTImporter] Add unittest case for friend decl import

Summary:
This change adds a matching test case for the recent bug fix to
VisitFriendDecl in ASTImporterLookup.cpp.

See https://reviews.llvm.org/D82882 for details.

Reviewers: martong, a.sidorin, shafik

Reviewed By: martong

Subscribers: rnkovacs, teemperor, cfe-commits, dkrupp

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83006