[ASTImporter] Fix infinite recursion on function import with struct definition in parameters

Summary:
Importing a function having a struct definition in the parameter list
causes a crash in the importer via infinite recursion. This patch avoids
the crash and reports such functions as not supported. Unit tests make
sure that normal struct definitions inside function bodies work normally
on the other hand and LLDB-like type imports also do.

Reviewers: a.sidorin, martong

Differential Revision: https://reviews.llvm.org/D47946

Patch by Zoltan Gera!

llvm-svn: 336898