[HIPIFY] Starts implicitly setting '-x cuda' by hipify-clang itself

+ No need in setting '-x cuda' for the user anymore;
+ Testing and Readme updated accordingly.

tests/hipify-clang/lit.cfg

@@ -60,7 +60,7 @@ if obj_root is not None:
     config.environment['PATH'] = path
 
 hipify_path = obj_root
-clang_args = "-x cuda -v --cuda-gpu-arch=sm_30 --cuda-path='%s'"
+clang_args = "-v --cuda-gpu-arch=sm_30 --cuda-path='%s'"
 
 if sys.platform in ['win32']:
     run_test_ext = ".bat"