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

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

## What changes were proposed in this pull request?

As srowen pointed in https://github.com/apache/spark/commit/609ba5f2b9fd89b1b9971d08f7cc680d202dbc7c#commitcomment-22221259, the previous tests are not proper.

This follow-up is going to fix the tests.

## 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 #18048 from viirya/SPARK-20399-follow-up.

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala

@@ -463,22 +463,30 @@ class ExpressionParserSuite extends PlanTest {
         assertEqual("'pattern\\\\\\%'", "pattern\\\\\\%", parser)
 
         // Escaped characters.
-        assertEqual("'\0'", "\u0000", parser) // ASCII NUL (X'00')
+        // Unescape string literal "'\\0'" for ASCII NUL (X'00') doesn't work
+        // when ESCAPED_STRING_LITERALS is enabled.
+        // It is parsed literally.
+        assertEqual("'\\0'", "\\0", parser)
 
         // Note: Single quote follows 1.6 parsing behavior when ESCAPED_STRING_LITERALS is enabled.
         val e = intercept[ParseException](parser.parseExpression("'\''"))
         assert(e.message.contains("extraneous input '''"))
 
-        assertEqual("'\"'", "\"", parser)     // Double quote
-        assertEqual("'\b'", "\b", parser)     // Backspace
-        assertEqual("'\n'", "\n", parser)     // Newline
-        assertEqual("'\r'", "\r", parser)     // Carriage return
-        assertEqual("'\t'", "\t", parser)     // Tab character
-
-        // Octals
-        assertEqual("'\110\145\154\154\157\041'", "Hello!", parser)
-        // Unicode
-        assertEqual("'\u0057\u006F\u0072\u006C\u0064\u0020\u003A\u0029'", "World :)", parser)
+        // The unescape special characters (e.g., "\\t") for 2.0+ don't work
+        // when ESCAPED_STRING_LITERALS is enabled. They are parsed literally.
+        assertEqual("'\\\"'", "\\\"", parser)   // Double quote
+        assertEqual("'\\b'", "\\b", parser)     // Backspace
+        assertEqual("'\\n'", "\\n", parser)     // Newline
+        assertEqual("'\\r'", "\\r", parser)     // Carriage return
+        assertEqual("'\\t'", "\\t", parser)     // Tab character
+
+        // The unescape Octals for 2.0+ don't work when ESCAPED_STRING_LITERALS is enabled.
+        // They are parsed literally.
+        assertEqual("'\\110\\145\\154\\154\\157\\041'", "\\110\\145\\154\\154\\157\\041", parser)
+        // The unescape Unicode for 2.0+ doesn't work when ESCAPED_STRING_LITERALS is enabled.
+        // They are parsed literally.
+        assertEqual("'\\u0057\\u006F\\u0072\\u006C\\u0064\\u0020\\u003A\\u0029'",
+          "\\u0057\\u006F\\u0072\\u006C\\u0064\\u0020\\u003A\\u0029", parser)
       } else {
         // Default behavior