[ScopInfo] Update Scop::addUserContext() to C++ interface

Summary: This patch updates `Scop::addUserContext()` function to the new C++ interface and replaces the `auto` keyword with explicit type wherever used in this function.

Reviewers: grosser, bollu, philip.pfaffe, chelini, Meinersbur

Reviewed By: grosser

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47438

llvm-svn: 333366