Fix stack-overflow in StringPiece
Fix stack-overflow in StringPiece

The compiler complains that StringPiece::operator<< leads to stack
overflows due to the infinite recursion (-Winfinite-recursion).

tsuchm/sentencepiece,google/sentencepiece,google/sentencepiece,tsuchm/sentencepiece,google/sentencepiece,tsuchm/sentencepiece,google/sentencepiece,tsuchm/sentencepiece