[ASTImporter] Use llvm::Expected and Error in the importer API

Summary:
This is the final phase of the refactoring towards using llvm::Expected
and llvm::Error in the ASTImporter API.
This involves the following:
- remove old Import functions which returned with a pointer,
- use the Import_New functions (which return with Err or Expected) everywhere
  and handle their return value
- rename Import_New functions to Import
This affects both Clang and LLDB.

Reviewers: shafik, teemperor, aprantl, a_sidorin, balazske, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits, lldb-commits

Tags: #clang, #lldb

Differential Revision: https://reviews.llvm.org/D61438

llvm-svn: 360760
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
index b310ddc..72ff3be 100644
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -40,7 +40,7 @@
     createVirtualFileIfNeeded(To, FromFileName,
                               From->getBufferForFile(FromFileName));
 
-    auto Imported = Importer.Import_New(Node);
+    auto Imported = Importer.Import(Node);
 
     if (Imported) {
       // This should dump source locations and assert if some source locations