[ASTImporter] Reorder fields after structure import is finished

We reorder declarations in RecordDecls because they may have another order
in the "to" context than they have in the "from" context. This may happen
e.g when we import a class like this:
   struct declToImport {
       int a = c + b;
       int b = 1;
       int c = 2;
   };
During the import of `a` we import first the dependencies in sequence,
thus the order would be `c`, `b`, `a`. We will get the normal order by
first removing the already imported members and then adding them in the
order as they apper in the "from" context.

Keeping field order is vital because it determines structure layout.

Reviewers: a_sidorin, shafik

Tags: #clang

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

llvm-svn: 366997