Port dotnet/runtime#31946 to release/uwp6.2 branch (#28015)

When string.Replace is given a target string with zero collation weight, it would enter an infinite loop. It is now changed so that the call to Replace terminates when such a condition is encountered.

Author: GrabYourPitchforks

src/mscorlib/shared/System/String.Manipulation.cs

@@ -967,7 +967,11 @@ private unsafe string ReplaceCore(string oldValue, string newValue, CultureInfo
             do
             {
                 index = ci.IndexOf(this, oldValue, startIndex, this.Length - startIndex, options, &matchLength);
-                if (index >= 0)
+
+                // There's the possibility that 'oldValue' has zero collation weight (empty string equivalent).
+                // If this is the case, we behave as if there are no more substitutions to be made.
+
+                if (index >= 0 && matchLength > 0)
                 {
                     // append the unmodified portion of string
                     result.Append(this, startIndex, index - startIndex);