[Diagnostics] Diagnose use of one or more return statements in a result builder body

lib/Sema/CSDiagnostics.cpp

@@ -7052,3 +7052,22 @@ bool ReferenceToInvalidDeclaration::diagnoseAsError() {
   emitDiagnosticAt(decl, diag::decl_declared_here, decl->getName());
   return true;
 }
+
+bool InvalidReturnInResultBuilderBody::diagnoseAsError() {
+  auto *closure = castToExpr<ClosureExpr>(getAnchor());
+
+  auto returnStmts = TypeChecker::findReturnStatements(closure);
+  assert(!returnStmts.empty());
+
+  auto loc = returnStmts.front()->getReturnLoc();
+  emitDiagnosticAt(loc, diag::result_builder_disabled_by_return, BuilderType);
+
+  // Note that one can remove all of the return statements.
+  {
+    auto diag = emitDiagnosticAt(loc, diag::result_builder_remove_returns);
+    for (auto returnStmt : returnStmts)
+      diag.fixItRemove(returnStmt->getReturnLoc());
+  }
+
+  return true;
+}