[stdlib] String.unicodeScalars: Add a _modify accessor

This will eliminate unnecessary CoW copies when calling mutating
Unicode scalar view methods directly through this property.

Author: lorentey

stdlib/public/core/StringUnicodeScalarView.swift

@@ -319,6 +319,14 @@ extension String {
   public var unicodeScalars: UnicodeScalarView {
     @inline(__always) get { return UnicodeScalarView(_guts) }
     @inline(__always) set { _guts = newValue._guts }
+
+    @_alwaysEmitIntoClient @inline(__always) // 5.7
+    _modify {
+      var view = self.unicodeScalars
+      self = ""
+      defer { self._guts = view._guts }
+      yield &view
+    }
   }
 }