stdlib: enable a check for a special case in Array.append(contentsOf:)

The check for the special case if the sequence is an Array was disabled in embedded swift.
It can be enabled because we now remove the cast after specialization.
Saves some code size when appending one array to another array.

Author: eeckstein

stdlib/public/core/Array.swift

@@ -1254,7 +1254,6 @@ extension Array: RangeReplaceableCollection {
 
     if _slowPath(writtenUpTo == buf.endIndex) {
 
-#if !$Embedded
       // A shortcut for appending an Array: If newElements is an Array then it's
       // guaranteed that buf.initialize(from: newElements) already appended all
       // elements. It reduces code size, because the following code
@@ -1264,7 +1263,6 @@ extension Array: RangeReplaceableCollection {
         _internalInvariant(remainder.next() == nil)
         return
       }
-#endif
 
       // there may be elements that didn't fit in the existing buffer,
       // append them in slow sequence-only mode