[ASTImporter] Reorder fields after structure import is finished

There are multiple reasons why field structures can be imported
in wrong order. The simplest is the ability of field initializers
and method bodies to refer fields not in order they are listed in.
Unfortunately, there is no clean solution for that currently
so I'm leaving a FIXME.

Differential Revision: https://reviews.llvm.org/D44100

llvm-svn: 345545