[ASTImporter] Added error handling for AST import.

Summary:
The goal of this change is to make the ASTImporter::Import functions return
llvm::Expected instead of the imported type.
As first part the ASTNodeImporter visit functions are updated to return with
llvm::Expected. Various `import` functions are added to ASTNodeImporter to
simplify the code and have a common place for interface towards ASTImporter
(from ASTNodeImporter). There is some temporary code that is needed before
ASTImporter is updated.

Reviewers: a.sidorin, a_sidorin, xazax.hun

Reviewed By: a_sidorin

Subscribers: dkrupp, Szelethus, rnkovacs, martong, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D51633

llvm-svn: 344783