message: Use formatv for the error instead of string stream.
diff: #include "llvm/IR/LLVMContext.h"
  #include "llvm/IR/Module.h"
  #include "llvm/IR/Type.h"
  +#include "llvm/Support/FormatVariadic.h"
  #include "llvm/Support/ToolOutputFile.h"
  using namespace mlir;
  @@ -268,11 +269,8 @@ llvm::Type *ModuleLowerer::convertType(Type type) {
  return convertVectorType(vectorType);
  MLIRContext *context = type.getContext();
  - std::string message;
  - llvm::raw_string_ostream os(message);
  - os << "unsupported type: ";
  - type.print(os);
  - context->emitError(UnknownLoc::get(context), os.str());
  + context->emitError(UnknownLoc::get(context),
  + llvm::formatv("unsupported type: {0}", type));
  return nullptr;
  }
  