[CMake] Use LIST_SEPARATOR rather than escaping in ExternalProject_Add

Escaping ; in list arguments passed to ExternalProject_Add doesn't seem
to be working in newer versions of CMake (see
https://public.kitware.com/Bug/view.php?id=16137 for more details). Use
a custom LIST_SEPARATOR instead which is the officially supported way.

Differential Revision: https://reviews.llvm.org/D40257

llvm-svn: 319264