[ASTImporter] Load external Decls when getting field index.

Summary:
At equality check of fields without name the index of fields is compared.
At determining the index of a field all fields of the parent context
should be loaded from external source to find the field at all.

Reviewers: a.sidorin, a_sidorin, r.stahl

Reviewed By: a.sidorin

Subscribers: martong, cfe-commits

Differential Revision: https://reviews.llvm.org/D49796

llvm-svn: 339226