Correct handling of 'u' in CLDR date format strings

It explicitly excludes having a two-digit special case like 'yy'.
Correct that in qlocale_mac.mm, add support in dateconverter.py
No current locale actually uses the 'u' format, so this makes no
change to data.

Change-Id: I16dfed2d3a7d2054b4b86f9a246bff297df9fc0a
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qlocale_mac.mm

@@ -344,7 +344,10 @@ static QVariant macToQtFormat(QStringView sys_fmt)
         case 'Y': // Year for Week-of-year calendars (1..n): 1..n = padded number
             break;
 
-        case 'u': // Extended Year (1..n): 2 = short year, 1 & 3..n = padded number
+        case 'u': // Extended Year (1..n), padded number.
+            // Explicitly has no special case for 'uu' as only the last two digits.
+            result += "yyyy"_L1;
+            break;
         case 'y': // Year (1..n): 2 = short year, 1 & 3..n = padded number
             // Qt only supports long (4) or short (2) year, use long for all others
             if (repeat == 2)

util/locale_database/dateconverter.py

@@ -100,6 +100,13 @@ class Converter (object):
         n = cls.__count_first(text)
         return ('z' if n < 3 else 'zzz'), n
 
+    @classmethod
+    def u(cls, text): # Extended year (numeric)
+        # Officially, 'u' is simply the full year number, zero-padded
+        # to the length of the field. Qt's closest to that is four-digit.
+        # It explicitly has no special case for two-digit year.
+        return 'yyyy', cls.__count_first(text)
+
     # U: Cyclic Year Name. Not supported
     @classmethod
     def v(cls, text): # Generic non-location format. Map to abbreviation.