Introduce StringBuilder.clear() extension

#KT-18910 Fixed

libraries/stdlib/common/src/kotlin/TextH.kt

@@ -129,6 +129,15 @@ public expect fun String.decapitalize(): String
 public expect fun CharSequence.repeat(n: Int): String
 
 
+/**
+ * Clears the content of this string builder making it empty.
+ *
+ * @sample samples.text.Strings.clearStringBuilder
+ */
+@SinceKotlin("1.3")
+public expect fun StringBuilder.clear(): StringBuilder
+
+
 /**
  * Returns a new string with all occurrences of [oldChar] replaced with [newChar].
  */