Left arm bowlers who turn the ball away from a left hand batsman or into a right hand batsman are called leg spinners.
This is how the ball will spin after it pitches.
But over time, the term Left-arm unorthodox spinners or Chinaman has become popular and is basically the norm now. However, it is not incorrect to use the term left arm leg spinners as few cricket writers still use this term.
We do not hear much of this term as there are very few crickets who fall under this category. The most famous ones being South African, Paul Adams and Australian, Brag Hogg.
Edit 1: Wiki quotes the origin of the term chinaman as below:
The name has its origins in a Test match played between the West Indies and England at Old Trafford, Manchester, in the year 1933. Elliss "Puss" Achong, a player of Chinese origin, was a left-arm orthodox spinner, playing for the West Indies at the time. According to folklore, Achong is said to have had Walter Robins stumped off a surprise delivery that spun into the right-hander from outside the off stump. As he walked back to the pavilion, Robins said to the umpire, "fancy being done by a bloody Chinaman!", leading to the popularity of the term in England, and subsequently, in the rest of the world.