[ASTImporter] Fix in ASTImporter::Import_New(const Decl *)

Make sure ASTImporter::Import_New(const Decl *) returns
a Expected<const Decl *> and not Expected<Decl *> to
make the clang/unittests/AST/ASTImporterTest.cpp compile
without the warning

 clang/unittests/AST/ASTImporterTest.cpp:117:12: error: no viable conversion from 'Expected<clang::Decl *>' to 'Expected<const clang::Decl *>'
    return Imported;

(I got the above when building with clang 3.6).

llvm-svn: 357985