[_CollectionsUtilities] Work around SR-14663

Unfortunate edge case: the new `initialize(fromContentsOf:)` from
SE-0370 is calling withContiguousStorageIfAvailable, so it is subject
to SR-14663.

This means we can either choose to continue maintaining the
workaround, or we can call the stdlib’s implementation, but not both.

Author: lorentey

Sources/_CollectionsUtilities/UnsafeMutableBufferPointer+SE-0370.swift

@@ -64,7 +64,7 @@ extension UnsafeMutableBufferPointer {
     fromContentsOf source: C
   ) -> Index
   where C.Element == Element {
-    let count = source.withContiguousStorageIfAvailable {
+    let count = source._withContiguousStorageIfAvailable_SR14663 {
       guard let sourceAddress = $0.baseAddress, !$0.isEmpty else {
         return 0
       }