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

cmake/Modules/CMakePolicy.cmake

@@ -0,0 +1,12 @@
+# CMake policy settings shared between LLVM projects
+
+# CMP0114: ExternalProject step targets fully adopt their steps.
+# New in CMake 3.19: https://cmake.org/cmake/help/latest/policy/CMP0114.html
+if(POLICY CMP0114)
+  cmake_policy(SET CMP0114 OLD)
+endif()
+# CMP0116: Ninja generators transform `DEPFILE`s from `add_custom_command()`
+# New in CMake 3.20. https://cmake.org/cmake/help/latest/policy/CMP0116.html
+if(POLICY CMP0116)
+  cmake_policy(SET CMP0116 OLD)
+endif()