Sema: Don't diagnose thrown error type that contains errors

If we have error types here, type resolution already diagnosed an
error; don't diagnose again.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -2545,7 +2545,7 @@ InterfaceTypeRequest::evaluate(Evaluator &eval, ValueDecl *D) const {
     if (thrownTy) {
       thrownTy = AFD->getThrownInterfaceType();
       ProtocolDecl *errorProto = Context.getErrorDecl();
-      if (thrownTy && errorProto) {
+      if (thrownTy && !thrownTy->hasError() && errorProto) {
         Type thrownTyInContext = AFD->mapTypeIntoContext(thrownTy);
         if (!checkConformance(thrownTyInContext, errorProto)) {
           SourceLoc loc;

test/decl/func/typed_throws.swift

@@ -1,5 +1,9 @@
 // RUN: %target-typecheck-verify-swift -swift-version 5 -module-name test
 
+// Don't complain about <<error type>> not conforming to Error
+func invalidThrownType() throws(DoesNotExist) {}
+// expected-error@-1 {{cannot find type 'DoesNotExist' in scope}}
+
 // expected-note@+1{{type declared here}}
 enum MyError: Error {
   case fail