Return StringBuilder back to cache when not making any replacements (#15677)

Author: bbowyersmyth

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

@@ -999,6 +999,7 @@ private unsafe String ReplaceCore(string oldValue, string newValue, CultureInfo
                     // small optimization,
                     // if we have not done any replacements,
                     // we will return the original string
+                    StringBuilderCache.Release(result);
                     return this;
                 }
                 else