[String] Use the new value in utf8 setter.

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stdlib/public/core/StringUTF8View.swift

@@ -241,11 +241,7 @@ extension String {
   @inlinable
   public var utf8: UTF8View {
     @inline(__always) get { return UTF8View(self._guts) }
-    set {
-      // TODO(String testing): test suite doesn't currenlty exercise this code at
-      // all, test it.
-      self = String(utf8._guts)
-    }
+    set { self = String(newValue._guts) }
   }
 
   /// A contiguously stored null-terminated UTF-8 representation of the string.

validation-test/stdlib/StringViews.swift

@@ -796,4 +796,28 @@ tests.test("String.UTF32View/BidirectionalCollection")
     test.unicodeScalars, test.string.unicodeScalars) { $0 == $1 }
 }
 
+tests.test("String View Setters") {
+  var string = "abcd🤠👨‍👨‍👦‍👦efg"
+
+  string.utf8 = winter.utf8
+  expectEqual(winter, string)
+  string.utf8 = summer.utf8
+  expectEqual(summer, string)
+
+  string.utf16 = winter.utf16
+  expectEqual(winter, string)
+  string.utf16 = summer.utf16
+  expectEqual(summer, string)
+
+  string.unicodeScalars = winter.unicodeScalars
+  expectEqual(winter, string)
+  string.unicodeScalars = summer.unicodeScalars
+  expectEqual(summer, string)
+
+  string = winter
+  expectEqual(winter, string)
+  string = summer
+  expectEqual(summer, string)
+}
+
 runAllTests()