Fix -Werror build

In file included from ../llvm-project/flang/lib/Lower/OpenMP/Clauses.cpp:9:
../llvm-project/flang/lib/Lower/OpenMP/Clauses.h:195:17: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
  195 |   return Clause{id, specific, source};
      |                 ^~~~~~~~~~~~
      |                 {           }