Fix rmm build (#7973)

- Optionally switch to c++17
- Use rmm CMake target.
- Workaround compiler errors.
- Fix GPUMetric inheritance.
- Run death tests even if it's built with RMM support.

Co-authored-by: jakirkham <jakirkham@gmail.com>

plugin/CMakeLists.txt

@@ -2,19 +2,6 @@ if (PLUGIN_DENSE_PARSER)
   target_sources(objxgboost PRIVATE ${xgboost_SOURCE_DIR}/plugin/dense_parser/dense_libsvm.cc)
 endif (PLUGIN_DENSE_PARSER)
 
-if (PLUGIN_RMM)
-  find_path(RMM_INCLUDE "rmm" HINTS "$ENV{RMM_ROOT}/include")
-  if (NOT RMM_INCLUDE)
-    message(FATAL_ERROR "Could not locate RMM library")
-  endif ()
-
-  message(STATUS "RMM: RMM_LIBRARY set to ${RMM_LIBRARY}")
-  message(STATUS "RMM: RMM_INCLUDE set to ${RMM_INCLUDE}")
-
-  target_include_directories(objxgboost PUBLIC ${RMM_INCLUDE})
-  target_compile_definitions(objxgboost PUBLIC -DXGBOOST_USE_RMM=1)
-endif (PLUGIN_RMM)
-
 if (PLUGIN_UPDATER_ONEAPI)
   add_library(oneapi_plugin OBJECT
     ${xgboost_SOURCE_DIR}/plugin/updater_oneapi/regression_obj_oneapi.cc