[cmake] Use DetectTestCompiler from openmp directory

Fix the DetectTestCompiler project use to reference the openmp source
tree, since the respective files were not copied to offload, and there
is no point in duplicating them.

Fixes llvm#90333

offload/cmake/OpenMPTesting.cmake

@@ -124,7 +124,7 @@ if (${OPENMP_STANDALONE_BUILD})
   # project is built which is too late for detecting the compiler...
   file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/DetectTestCompiler)
   execute_process(
-    COMMAND ${CMAKE_COMMAND} -G${CMAKE_GENERATOR} ${CMAKE_CURRENT_LIST_DIR}/DetectTestCompiler
+    COMMAND ${CMAKE_COMMAND} -G${CMAKE_GENERATOR} ${CMAKE_CURRENT_SOURCE_DIR}/../openmp/cmake/DetectTestCompiler
       -DCMAKE_C_COMPILER=${OPENMP_TEST_C_COMPILER}
       -DCMAKE_CXX_COMPILER=${OPENMP_TEST_CXX_COMPILER}
     WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/DetectTestCompiler