[Bug c/63398] New: Cilk errors out incorrectly for spawn inside statement expressions

["ak at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63398

            Bug ID: 63398
           Summary: Cilk errors out incorrectly for spawn inside statement
                    expressions
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ak at gcc dot gnu.org

Like in:

void f2();
int f()
{
        return ({ _Cilk_spawn f2(); 0; });
} 

and some other places that use contains_silk_spawn_stmt to check for errors.
But that should be legal.

The problem is the walk_tree in contains_silk_spawn_statement doesn't stop
recursing into the statement.