Use NEW behavior for CMake policy CMP0051
Use NEW behavior for CMake policy CMP0051

We need to move LLVM in ROOT to use the new behavior of policy CMP0051
as it will be removed soon and the warning below will become an error:

CMake Deprecation Warning at interpreter/llvm/src/CMakeLists.txt:15 (cmake_policy):
  The OLD behavior for policy CMP0051 will be removed from a future version
  of CMake.

  The cmake-policies(7) manual explains that the OLD behaviors of all
  policies are deprecated and that a policy should be set to OLD only under
  specific short-term circumstances.  Projects should be ported to the NEW
  behavior and not rely on setting a policy to OLD.

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