[Comgr][test] Add -nostdinc++ to compile_hip_with_libcxx_test

amd/comgr/test/CMakeLists.txt

@@ -246,6 +246,6 @@ add_comgr_test(compile_source_to_executable c)
 add_comgr_test(name_expression_map_test c)
 add_comgr_test(compile_hip_test c)
 add_comgr_test(compile_hip_to_relocatable c)
-#add_comgr_test(compile_hip_with_libcxx_test c)
+add_comgr_test(compile_hip_with_libcxx_test c)
 add_comgr_test(mangled_names_hip_test c)
 #add_comgr_test(unbundle_hip_test c)

amd/comgr/test/compile_hip_with_libcxx_test.c

@@ -82,12 +82,14 @@ int main(int Argc, char *Argv[]) {
   amd_comgr_action_info_t ActionInfo;
   amd_comgr_status_t Status;
 
-  // Compile options: embedded libc++ headers are mapped to clang's default
-  // include locations via VFS and injected as a fallback (-idirafter).
-  // No explicit -I flags needed.
+  // Embedded libc++ headers are VFS-mapped to clang's default include
+  // locations and injected at -idirafter priority. -nostdinc++ disables
+  // system C++ header lookup so the embedded headers are exercised
+  // (otherwise system libstdc++, when present, wins).
   const char *CompileOptions[] = {
       "-std=c++17",
-      "-nogpuinc"                  // Don't use GPU-specific includes
+      "-nogpuinc",                 // Don't use GPU-specific includes
+      "-nostdinc++"                // Use only the embedded libc++ headers
   };
   size_t CompileOptionsCount =
       sizeof(CompileOptions) / sizeof(CompileOptions[0]);