[SPARK-4205][SQL] DSL expressions with a literal on the left hand side

Currently the DSL expression parser does not recognize expressions which begin with a literal, such as:
`0 < 'x`.

This PR adds a new implicit class which allows such expressions to be included in the DSL.  Due to conflicts with scalatest, which converts  `===` and `!==` in its `assert` macro, these two operators can not be used by the new class.  It adds two new operators `-=-` and `!=-` which behave identically to `===` and `!==` within the DSL, but can also be used in expressions which start with a literal.

The new class replaces the comment: `// TODO more implicit class for literal?`
I think this may be what the author had in mind.