[ASTImporter] Fix import of unnamed structs

Summary:
D48773 simplified ASTImporter nicely, but it introduced a new error: Unnamed
structs are not imported correctly, if they appear in a recursive context.
This patch provides a fix for structural equivalency.

Reviewers: a.sidorin, a_sidorin, balazske, gerazo

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D49296

llvm-svn: 337267