[SPARK-13995][SQL] Extract correct IsNotNull constraints for Expression

## What changes were proposed in this pull request?

JIRA: https://issues.apache.org/jira/browse/SPARK-13995

We infer relative `IsNotNull` constraints from logical plan's expressions in `constructIsNotNullConstraints` now. However, we don't consider the case of (nested) `Cast`.

For example:

    val tr = LocalRelation('a.int, 'b.long)
    val plan = tr.where('a.attr === 'b.attr).analyze

Then, the plan's constraints will have `IsNotNull(Cast(resolveColumn(tr, "a"), LongType))`, instead of `IsNotNull(resolveColumn(tr, "a"))`. This PR fixes it.

Besides, as `IsNotNull` constraints are most useful for `Attribute`, we should do recursing through any `Expression` that is null intolerant and construct `IsNotNull` constraints for all `Attribute`s under these Expressions.

For example, consider the following constraints:

    val df = Seq((1,2,3)).toDF("a", "b", "c")
    df.where("a + b = c").queryExecution.analyzed.constraints

The inferred isnotnull constraints should be isnotnull(a), isnotnull(b), isnotnull(c), instead of isnotnull(a + c) and isnotnull(c).

## How was this patch tested?

Test is added into `ConstraintPropagationSuite`.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>

Closes #11809 from viirya/constraint-cast.