Fix broken test -- CTFE on an uninitialized variable

This shouldn't compile. In most cases, a const variable with no initializer
could be set by 'this' (if it is a struct member) or by static this (if it
is a global). We could allow this particular case (a local variable in a
static function) to compile, but there doesn't seem to be much point in
introducing that corner case.

test/runnable/xtest46.d

@@ -1731,7 +1731,7 @@ struct Foo93
 
 void test93()
 {
-    const Foo93 bar;
+    const Foo93 bar = Foo93();
     enum bla = bar.foo();
     assert(bla == 2);
 }