HIP: Fix default flags for Debug configuration

Pass `-g` to enable debugging, not `-O` for optimizations.  This was
probably a typo in commit b50bfc89 (HIP: Add language to CMake,
2020-08-28, v3.21.0-rc1~66^2~4).

Fixes: #26823

Modules/Compiler/Clang-HIP.cmake

@@ -9,7 +9,7 @@ set(_CMAKE_HIP_RDC_FLAG "-fgpu-rdc")
 if(NOT "x${CMAKE_HIP_SIMULATE_ID}" STREQUAL "xMSVC")
   set(CMAKE_HIP_COMPILE_OPTIONS_VISIBILITY_INLINES_HIDDEN "-fvisibility-inlines-hidden")
 
-  string(APPEND CMAKE_HIP_FLAGS_DEBUG_INIT " -O")
+  string(APPEND CMAKE_HIP_FLAGS_DEBUG_INIT " -g")
 endif()
 
 set(CMAKE_HIP_RUNTIME_LIBRARY_DEFAULT "SHARED")