Add @preconcurrency to toplevel vars

Top-level global variables should have the `@predatesConcurrency
@MainActor` behaviour. This allows them to be somewhat concurrency safe
while still working with old code.

Author: etcwilde

lib/AST/Decl.cpp

@@ -733,10 +733,16 @@ bool Decl::preconcurrency() const {
   if (isa<ClangModuleUnit>(getDeclContext()->getModuleScopeContext()))
     return true;
 
+  // Variables declared in top-level code are @_predatesConcurrency
+  if (const VarDecl *var = dyn_cast<VarDecl>(this)) {
+    const LangOptions &langOpts = getASTContext().LangOpts;
+    return !langOpts.isSwiftVersionAtLeast(6) &&
+           langOpts.EnableExperimentalAsyncTopLevel && var->isTopLevelGlobal();
+  }
+
   return false;
 }
 
-
 Expr *AbstractFunctionDecl::getSingleExpressionBody() const {
   assert(hasSingleExpressionBody() && "Not a single-expression body");
   auto braceStmt = getBody();

lib/Sema/TypeCheckConcurrency.cpp

@@ -3516,7 +3516,7 @@ ActorIsolation ActorIsolationRequest::evaluate(
       if (Type mainActor = ctx.getMainActorType())
         return inferredIsolation(
             ActorIsolation::forGlobalActor(mainActor,
-                                           /*unsafe=*/false));
+                                           /*unsafe=*/var->preconcurrency()));
     }
     if (auto isolation = getActorIsolationFromWrappedProperty(var))
       return inferredIsolation(isolation);