ZLIB can not be configured with zlib-ng config files

https://github.com/llvm/llvm-project/blob/d08cbc1cdd7b73e9a582f5602e8ca4829decab8c/llvm/cmake/config-ix.cmake#L171C1-L182C10

```cmake
if(ZLIB_FOUND)
# Check if zlib we found is usable; for example, we may have found a 32-bit
# library on a 64-bit system which would result in a link-time failure.
cmake_push_check_state()
list(APPEND CMAKE_REQUIRED_INCLUDES ${ZLIB_INCLUDE_DIRS})
list(APPEND CMAKE_REQUIRED_LIBRARIES ${ZLIB_LIBRARY})
check_symbol_exists(compress2 zlib.h HAVE_ZLIB)
cmake_pop_check_state()
if(LLVM_ENABLE_ZLIB STREQUAL FORCE_ON AND NOT HAVE_ZLIB)
message(FATAL_ERROR "Failed to configure zlib")
endif()
endif()
```

The old CMake ZLIB module variables `ZLIB_INCLUDE_DIRS` and `ZLIB_LIBRARY` are not availiable with modern CMake config files from [zlib-ng](https://github.com/zlib-ng/zlib-ng). If you configure with `CMAKE_FIND_PACKAGE_PREFER_CONFIG=ON` and zlib-ng (in compatebility mode) the configuration fails.

Note that with modern CMake config files there is no need for the check in the first place.

Start in llvm/cmake/config-ix.cmake at the linked ZLIB usability check. Reproduce configuration with CMAKE_FIND_PACKAGE_PREFER_CONFIG=ON and zlib-ng in compatibility mode, then verify that modern zlib-ng config files configure successfully without relying on the unavailable legacy variables.

Written by the indexing model from the issue text.