[SPARK-17205] Literal.sql should handle Infinity and NaN

This patch updates `Literal.sql` to properly generate SQL for `NaN` and `Infinity` float and double literals: these special values need to be handled differently from regular values, since simply appending a suffix to the value's `toString()` representation will not work for these values.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #14777 from JoshRosen/SPARK-17205.