cpp11-migrate: Tidying up

* Some file headers were missing for files in Core/
* Some headers were included but not necessary
* CMakeLists.txt was linking in LLVMSupport even though CMakeLists in subdirs
  were linking it in too.
* StringRefisation of constructors of types in FileOverrides.h
* Other misc cleanups

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