[SPARK-20399][SQL] Add a config to fallback string literal parsing consistent...

[SPARK-20399][SQL] Add a config to fallback string literal parsing consistent with old sql parser behavior

## What changes were proposed in this pull request?

The new SQL parser is introduced into Spark 2.0. All string literals are unescaped in parser. Seems it bring an issue regarding the regex pattern string.

The following codes can reproduce it:

    val data = Seq("\u0020\u0021\u0023", "abc")
    val df = data.toDF()

    // 1st usage: works in 1.6
    // Let parser parse pattern string
    val rlike1 = df.filter("value rlike '^\\x20[\\x20-\\x23]+$'")
    // 2nd usage: works in 1.6, 2.x
    // Call Column.rlike so the pattern string is a literal which doesn't go through parser
    val rlike2 = df.filter($"value".rlike("^\\x20[\\x20-\\x23]+$"))

    // In 2.x, we need add backslashes to make regex pattern parsed correctly
    val rlike3 = df.filter("value rlike '^\\\\x20[\\\\x20-\\\\x23]+$'")

Follow the discussion in #17736, this patch adds a config to fallback to 1.6 string literal parsing and mitigate migration issue.

## How was this patch tested?

Jenkins tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #17887 from viirya/add-config-fallback-string-parsing.