Harmonize cmake_policy() across standalone builds of all projects

Move `cmake_policy()` settings from `llvm/CMakeLists.txt` into a shared
`cmake/modules/CMakePolicy.cmake`.  Include it from all relevant
projects that support standalone builds, in order to ensure that
the policies are consistently set whether they are built in-tree
or stand-alone.

Differential Revision: https://reviews.llvm.org/D136572