stdlib: make Array.count an array semantic function

So far the semantic annotation was just added to the internal `_getCount` function.
But for FixedArray it is also required to add the `@_semantics("array.get_count")` to the public API.

Author: eeckstein

stdlib/public/core/Array.swift

@@ -806,6 +806,7 @@ extension Array: RandomAccessCollection, MutableCollection {
 
   /// The number of elements in the array.
   @inlinable
+  @_semantics("array.get_count")
   public var count: Int {
     return _getCount()
   }