[ASTImporter] Import implicit methods of existing class.

Summary:
When an already existing class is encountered during import,
check if it has implicit methods that are missing in the existing one,
and import these.
The to-be-imported code may use the same class in different way than the
existing (before the import) code. This may result in that there are
implicit methods that are not generated for the existing code.

Reviewers: a.sidorin, a_sidorin

Reviewed By: a_sidorin

Subscribers: a_sidorin, martong, cfe-commits

Differential Revision: https://reviews.llvm.org/D49245

llvm-svn: 337162