[ASTImporter] Added Import functions for transition to new API.

Summary:
These Import_New functions should be used in the ASTImporter,
and the old Import functions should not be used. Later the
Import_New should be renamed to Import again and the old Import
functions must be removed. But this can happen only after LLDB
was updated to use the new Import interface.

This commit is only about introducing the new Import_New
functions. These are not implemented now, only calling the old
Import ones.

Reviewers: shafik, rsmith, a_sidorin, a.sidorin

Reviewed By: a_sidorin

Subscribers: spyffe, a_sidorin, gamesh411, shafik, rsmith, dkrupp, martong, Szelethus, cfe-commits

Differential Revision: https://reviews.llvm.org/D53751

llvm-svn: 347685