cpp11-migrate: Make a VirtualFilesHelper class for the unit tests.

This change makes it easier for new unit tests to create virtual files.

Author: Guillaume Papin <guillaume.papin@epitech.eu>



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@186842 91177308-0d34-0410-b5e6-96231b3b80d8