Fix Data.count’s setter

Data provides a settable `count` property. Its expected behavior is undocumented, but based on the implementation, it is intended to zero-extend (or truncate) the collection to the specified length.

This does not work correctly if we start with an empty Data and we try to increase the count by a small integer:

```
import Foundation

var d = Data()
d.count = 2
print(d.count) // ⟹ 0 ⁉️

d.count = 100
print(d.count) // ⟹ 100 ✓
```

It looks like this bug was introduced with the Data overhaul that shipped in Swift 5.

(This issue was uncovered by swiftlang/swift#28918.)

rdar://58134026

Author: lorentey

Foundation/Data.swift

@@ -1440,7 +1440,7 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
                         if newValue == 0 {
                             return nil
                         } else if InlineData.canStore(count: newValue) {
-                            return .inline(InlineData())
+                            return .inline(InlineData(count: newValue))
                         } else if InlineSlice.canStore(count: newValue) {
                             return .slice(InlineSlice(count: newValue))
                         } else {

TestFoundation/TestNSData.swift

@@ -528,6 +528,8 @@ class TestNSData: LoopbackServerTest {
             ("test_replaceSubrangeReferencingMutable", test_replaceSubrangeReferencingMutable),
             ("test_replaceSubrangeReferencingImmutable", test_replaceSubrangeReferencingImmutable),
             ("test_rangeOfSlice", test_rangeOfSlice),
+            ("test_Data_increaseCount", test_Data_increaseCount),
+            ("test_Data_decreaseCount", test_Data_decreaseCount),
         ]
     }
 
@@ -4528,5 +4530,52 @@ extension TestNSData {
         let decodedData = NSData(coder: unarchiver)
         XCTAssertEqual(data, decodedData)
     }
+
+    func test_Data_increaseCount() {
+         guard #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) else { return }
+         let initials: [Range<UInt8>] = [
+             0..<0,
+             0..<2,
+             0..<4,
+             0..<8,
+             0..<16,
+             0..<32,
+             0..<64
+         ]
+         let diffs = [0, 1, 2, 4, 8, 16, 32]
+         for initial in initials {
+             for diff in diffs {
+                 var data = Data(initial)
+                 data.count += diff
+                 expectEqualSequence(
+                     Array(initial) + Array(repeating: 0, count: diff),
+                     data)
+             }
+         }
+     }
+
+     func test_Data_decreaseCount() {
+         guard #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) else { return }
+         let initials: [Range<UInt8>] = [
+             0..<0,
+             0..<2,
+             0..<4,
+             0..<8,
+             0..<16,
+             0..<32,
+             0..<64
+         ]
+         let diffs = [0, 1, 2, 4, 8, 16, 32]
+         for initial in initials {
+             for diff in diffs {
+                 guard initial.count >= diff else { continue }
+                 var data = Data(initial)
+                 data.count -= diff
+                 expectEqualSequence(
+                     initial.dropLast(diff),
+                     data)
+             }
+         }
+     }
 }