SIL: Print ErrorTypes with @error_type prefix to avoid confusion

In diagnostics, we print wrapped ErrorTypes as their original type.
This is confusing when debugging SIL issues though, where we don't
expect to see ErrorTypes. Print them as '@error_type <origtype>'
instead.

Author: slavapestov

lib/AST/ASTPrinter.cpp

@@ -3780,8 +3780,11 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
   }
 
   void visitErrorType(ErrorType *T) {
-    if (auto originalType = T->getOriginalType())
+    if (auto originalType = T->getOriginalType()) {
+      if (Options.PrintInSILBody)
+        Printer << "@error_type ";
       visit(originalType);
+    }
     else
       Printer << "<<error type>>";
   }