NSCalendar: Change the type of force casting in func _symbols(_:) -> [String].

Despite `CFDateFormatterCopyProperty(_:_:)` returns `CFArrayRef`, the compiler on Linux assume that the type is `_NSCFArray` and fails to cast even though they are compatible. See https://bugs.swift.org/browse/SR-10638 .

Author: YOCKOW

Foundation/NSCalendar.swift

@@ -329,7 +329,7 @@ open class NSCalendar : NSObject, NSCopying, NSSecureCoding {
     private func _symbols(_ key: CFString) -> [String] {
         let dateFormatter = CFDateFormatterCreate(kCFAllocatorSystemDefault, locale?._cfObject, kCFDateFormatterNoStyle, kCFDateFormatterNoStyle)
         CFDateFormatterSetProperty(dateFormatter, kCFDateFormatterCalendarKey, _cfObject)
-        let result = (CFDateFormatterCopyProperty(dateFormatter, key) as! CFArray)._swiftObject
+        let result = (CFDateFormatterCopyProperty(dateFormatter, key) as! NSArray)._swiftObject
         return result.map {
             return ($0 as! NSString)._swiftObject
         }

TestFoundation/TestCalendar.swift

@@ -26,6 +26,7 @@ class TestCalendar: XCTestCase {
             ("test_currentCalendarRRstability", test_currentCalendarRRstability),
             ("test_hashing", test_hashing),
             ("test_dateFromDoesntMutate", test_dateFromDoesntMutate),
+            ("test_sr10638", test_sr10638),
         ]
     }
 
@@ -254,6 +255,12 @@ class TestCalendar: XCTestCase {
         XCTAssertEqual(calendarCopy.timeZone, calendar.timeZone)
         XCTAssertEqual(calendarCopy, calendar)
     }
+    
+    func test_sr10638() {
+        // https://bugs.swift.org/browse/SR-10638
+        let cal = Calendar(identifier: .gregorian)
+        XCTAssertGreaterThan(cal.eraSymbols.count, 0)
+    }
 }
 
 class TestNSDateComponents: XCTestCase {