Fix datetime parsing for half day when hour is not specified (#8349)

Summary:
In both Spark and Presto, half day specifier (AM or PM) takes effect even when
hour is not specified. For example, parsing ("2018-04-28 59:30PM",
"yyyy-MM-dd mm:ss a") returns the timestamp of 2018-04-28 12:59:30, but
Velox returns 2018-04-28 00:59:30, which has 12-hour difference.

Pull Request resolved: #8349

Reviewed By: xiaoxmeng

Differential Revision: D52693228

Pulled By: mbasmanova

fbshipit-source-id: 9454e4a95d64e095ed55bdece6af307029c02bf8

velox/functions/lib/DateTimeFormatter.cpp

@@ -61,7 +61,7 @@ struct Date {
 
   bool isClockHour = false; // Whether most recent hour specifier is clockhour
   bool isHourOfHalfDay =
-      false; // Whether most recent hour specifier is of half day.
+      true; // Whether most recent hour specifier is of half day.
 
   std::vector<int32_t> dayOfMonthValues;
   std::vector<int32_t> dayOfYearValues;

velox/functions/lib/tests/DateTimeFormatterTest.cpp

@@ -1004,6 +1004,17 @@ TEST_F(JodaDateTimeFormatterTest, parseHalfOfDay) {
   EXPECT_EQ(
       util::fromTimestampString("1970-01-01 12:00:00"),
       parseJoda("1 AM 12", "h a H").timestamp);
+
+  // Half of day still has effect even though hour or clockhour is not provided.
+  EXPECT_EQ(
+      util::fromTimestampString("1970-01-01 12:00:00"),
+      parseJoda("PM", "a").timestamp);
+  EXPECT_EQ(
+      util::fromTimestampString("1970-01-01 12:11:11"),
+      parseJoda("11:11 PM", "mm:ss a").timestamp);
+  EXPECT_EQ(
+      util::fromTimestampString("2018-04-28 12:59:30"),
+      parseJoda("2018-04-28 59:30 PM", "yyyy-MM-dd mm:ss a").timestamp);
 }
 
 TEST_F(JodaDateTimeFormatterTest, parseMinute) {