AST: Spot fix for latent issue with thrown error in transformWithPosition()

We probably don't have a way to exercise this, but the thrown error
type appears in the same position as the function, instead of being
invariant. That is, we wish to allow converting `() throws(E) -> ()`
into `() throws(F) -> ()` when E converts to F.

Author: slavapestov

lib/AST/Type.cpp

@@ -5045,8 +5045,7 @@ case TypeKind::Id:
     // Transform the thrown error.
     Type thrownError;
     if (Type origThrownError = function->getThrownError()) {
-      thrownError = origThrownError.transformWithPosition(
-          TypePosition::Invariant, fn);
+      thrownError = origThrownError.transformWithPosition(pos, fn);
       if (!thrownError)
         return Type();