Allow actor isolation in top-level code variables

Don't crash when we're SILGen'ing accesses to globals in top-level code.
The address is found in the top-level main function because we generate
the initialization code directly. In the case when we're generating the
body of the implicit main function, the presence of the address does not
indicate that the variable is local.

Author: etcwilde

lib/SILGen/SILGenLValue.cpp

@@ -2815,7 +2815,8 @@ void LValue::addNonMemberVarComponent(SILGenFunction &SGF, SILLocation loc,
       if (!address) {
         address = SGF.emitGlobalVariableRef(Loc, Storage, ActorIso);
       } else {
-        assert(!ActorIso && "local var should not be actor isolated!");
+        assert((!ActorIso || Storage->isTopLevelGlobal()) &&
+               "local var should not be actor isolated!");
       }
       assert(address.isLValue() &&
              "physical lvalue decl ref must evaluate to an address");