[ASTImporter] Fix import of objects with anonymous types

Summary:
Currently, anonymous types are merged into the same redecl chain even if they
are structurally inequivalent. This results that global objects are not
imported, if there are at least two global objects with different anonymous
types. This patch provides a fix.

Reviewers: a.sidorin, balazske, r.stahl

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D48773

llvm-svn: 336332