message: Initialize union to avoid -Wmissing-field-initializers warning.
  Reported by clang-6.
diff: @@ -320,7 +320,7 @@ struct ConversionPatternRewriterImpl {
  /// contains the information necessary to undo this action.
  struct BlockAction {
  static BlockAction getSplit(Block *block, Block *originalBlock) {
  - BlockAction action{BlockActionKind::Split, block};
  + BlockAction action{BlockActionKind::Split, block, {}};
  action.originalBlock = originalBlock;
  return action;
  }
  @@ -328,7 +328,7 @@ struct ConversionPatternRewriterImpl {
  return {BlockActionKind::Move, block, {originalPos}};
  }
  static BlockAction getTypeConversion(Block *block) {
  - return BlockAction{BlockActionKind::TypeConversion, block};
  + return BlockAction{BlockActionKind::TypeConversion, block, {}};
  }
  // The action kind.
  