[CMake] Set compatibility policy for lists in LLVM-Config.cmake.

Fixes "dependent projects fail to build with cmake 3.25 and llvm 16-rc4".

When building [latest `iovisor/bcc`](iovisor/bcc@1ea0390), `cmake` complains about a change in the behavior of operator `IN_LIST`:
```console
CMake Warning (dev) at /usr/local/lib/cmake/llvm/LLVM-Config.cmake:230 (if):
  Policy CMP0057 is not set: Support new IN_LIST if() operator.  Run "cmake
  --help-policy CMP0057" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.

  IN_LIST will be interpreted as an operator when the policy is set to NEW.
  Since the policy is not set the OLD behavior will be used.
Call Stack (most recent call first):
  cmake/clang_libs.cmake:32 (llvm_map_components_to_libnames)
  src/cc/CMakeLists.txt:132 (include)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Error at /usr/local/lib/cmake/llvm/LLVM-Config.cmake:230 (if):
  if given arguments:

    "engine" "IN_LIST" "link_components"

  Unknown arguments specified
Call Stack (most recent call first):
  cmake/clang_libs.cmake:32 (llvm_map_components_to_libnames)
  src/cc/CMakeLists.txt:132 (include)
```

This patch addresses this issue by setting the aforementioned policy in `LLVM-Config.cmake`.

Reviewed By: mgorny, thesamesam

Differential Revision: https://reviews.llvm.org/D150884

Closes: #61436
(cherry picked from commit 75a0502)

llvm/cmake/modules/LLVM-Config.cmake

@@ -1,3 +1,6 @@
+cmake_policy(PUSH)
+cmake_policy(SET CMP0057 NEW)
+
 function(get_system_libs return_var)
   message(AUTHOR_WARNING "get_system_libs no longer needed")
   set(${return_var} "" PARENT_SCOPE)
@@ -343,3 +346,5 @@ function(explicit_map_components_to_libraries out_libs)
   endforeach(c)
   set(${out_libs} ${result} PARENT_SCOPE)
 endfunction(explicit_map_components_to_libraries)
+
+cmake_policy(POP)