[stdlib] make Sequence's _copyContents call into a non-protocol function

Author: glessard

stdlib/public/core/Sequence.swift

@@ -1140,6 +1140,13 @@ extension Sequence {
   @inlinable
   public __consuming func _copyContents(
     initializing buffer: UnsafeMutableBufferPointer<Element>
+  ) -> (Iterator,UnsafeMutableBufferPointer<Element>.Index) {
+    return _copySequenceContents(initializing: buffer)
+  }
+
+  @inlinable
+  internal __consuming func _copySequenceContents(
+    initializing buffer: UnsafeMutableBufferPointer<Element>
   ) -> (Iterator,UnsafeMutableBufferPointer<Element>.Index) {
     var it = self.makeIterator()
     guard var ptr = buffer.baseAddress else { return (it,buffer.startIndex) }