[llvm] [OFFLOAD] Add plugin with support for Intel oneAPI Level Zero (PR #158900)

[Piotr Balcer via llvm-commits]

pbalcer wrote:

I managed to get sycl-ls past iterating the platforms, but it's failing on name info query:
```
$ UR_ADAPTERS_FORCE_LOAD=~/llvm/build/lib/libur_adapter_offload.so ./sycl-ls --verbose

...
   <--- urDeviceRelease(.hDevice = 0x55a95bdef6b0) -> UR_RESULT_SUCCESS;
[offload:gpu][offload:0]    ---> urDeviceGetInfo
   <--- urDeviceGetInfo(.hDevice = 0x55a95bdef6b0, .propName = UR_DEVICE_INFO_VERSION, .propSize = 0, .pPropValue = nullptr, .pPropSizeRet = 0x7fff5f2741a8 (1)) -> UR_RESULT_SUCCESS;
   ---> urDeviceGetInfo
   <--- urDeviceGetInfo(.hDevice = 0x55a95bdef6b0, .propName = UR_DEVICE_INFO_VERSION, .propSize = 1, .pPropValue = 0x7fff5f2741e0 (), .pPropSizeRet = nullptr) -> UR_RESULT_SUCCESS;
   ---> urDeviceGetInfo
---> olGetDeviceInfoSize(.Device = 0x55a95be219a0, .PropName = OL_DEVICE_INFO_NAME, .PropSizeRet = 0x7fff5f274078 (0))-> OL_ERRC_UNIMPLEMENTED
     *Error Details* OL_DEVICE_INFO_NAME: plugin did not provide a response for this information 
   <--- urDeviceGetInfo(.hDevice = 0x55a95bdef6b0, .propName = UR_DEVICE_INFO_NAME, .propSize = 0, .pPropValue = nullptr, .pPropSizeRet = 0x7fff5f274078 (0)) -> UR_RESULT_ERROR_UNKNOWN;
SYCL Exception encountered: offload backend failed with error: 2147483646 (UR_RESULT_ERROR_UNKNOWN)

   ---> urDeviceRelease
   <--- urDeviceRelease(.hDevice = 0x55a95bdef6b0) -> UR_RESULT_SUCCESS;
   ---> urAdapterRelease
   <--- urAdapterRelease(.hAdapter = 0x55a95a02b040) -> UR_RESULT_SUCCESS;
```
This is likely related @RossBrunton earlier comment https://github.com/llvm/llvm-project/pull/158900#discussion_r2362562819

https://github.com/llvm/llvm-project/pull/158900