Always allow ({ }) in the ctrl-expr of _Generic

Always allow ({ }) in the ctrl-expr of _Generic

tcc would reject e.g.,
    void f(){ struct {_Bool x:_Generic(({0;}),default:1);} my_x; }
with `expected constant`. This patch makes it accept it.

(The patch also makes tcc's _Generic a little more "generic" than that
 of gcc and clang in that that tcc now also accepts
`struct {_Bool x:_Generic(({0;}),default:1);} my_x;` in file scope
while gcc and clang don't, but I think there's no harm in that
and gcc and clang might as well accept it in filescope too, given
that they have no problem with
e.g., `/*filescope:*/int x=1, y=2, z=_Generic(x+y, int:3);`)