[5.9] Try using withContiguousStorageIfAvailable in RangeReplaceableCollection.append(contentsOf:) before falling back to a slow element-by-element loop (#66838)

* Try using withContiguousStorageIfAvailable in RangeReplaceableCollection.append(contentsOf:) before falling back to a slow element-by-element loop. Fixes rdar://109059874

(cherry picked from commit 465aa22)

* _onFastPath, in inlineable code, will end up applying to the caller. We don't want that, so omit it

(cherry picked from commit 5efc2da)

* Don't reserveCapacity in append(contentsOf:), it breaks API guarantees of asymptotic complexity

(cherry picked from commit 476c472)

Author: Catfish-Man

stdlib/public/core/RangeReplaceableCollection.swift

@@ -453,11 +453,15 @@ extension RangeReplaceableCollection {
   @inlinable
   public mutating func append<S: Sequence>(contentsOf newElements: __owned S)
     where S.Element == Element {
-
-    let approximateCapacity = self.count + newElements.underestimatedCount
-    self.reserveCapacity(approximateCapacity)
-    for element in newElements {
-      append(element)
+    
+    let done:Void? = newElements.withContiguousStorageIfAvailable {
+      replaceSubrange(endIndex..<endIndex, with: $0)
+    }
+      
+    if done == nil {
+      for element in newElements {
+        append(element)
+      }
     }
   }