Introduced support of JDK15+

`java.lang.CharSequence` has `isEmpty` method since JDK 15 that makes impossibly
to compile scala by JDK15.

Fix it on the way that allows to compiled with JDK15 and before JDK15.

Link to the issue: scala/bug#12172

src/library/scala/collection/mutable/StringBuilder.scala

@@ -468,6 +468,12 @@ final class StringBuilder(val underlying: java.lang.StringBuilder) extends Abstr
    *  @return             the last applicable index where target occurs, or -1 if not found.
    */
   def lastIndexOf(str: String, fromIndex: Int): Int = underlying.lastIndexOf(str, fromIndex)
+
+  /** Tests whether this builder is empty.
+   *
+   *  @return  `true` if this builder contains nothing, `false` otherwise.
+   */
+  override def isEmpty: Boolean = underlying.length() == 0
 }
 
 object StringBuilder {

src/reflect/scala/reflect/api/Names.scala

@@ -121,6 +121,15 @@ trait Names {
     /** The encoded name, still represented as a name.
      */
     def encodedName: Name
+
+    /** The length of this name. */
+    def length: Int
+
+    /** Tests whether this name is empty. */
+    def isEmpty: Boolean
+
+    /** Tests whether this name is not empty. */
+    def nonEmpty: Boolean
   }
 
   /** Create a new term name.

src/reflect/scala/reflect/internal/Names.scala

@@ -207,9 +207,9 @@ trait Names extends api.Names {
     def next: Name with ThisNameType
 
     /** The length of this name. */
-    final def length: Int = len
-    final def isEmpty = length == 0
-    final def nonEmpty = !isEmpty
+    override final def length: Int = len
+    override final def isEmpty: Boolean = length == 0
+    override final def nonEmpty: Boolean = !isEmpty
 
     def nameKind: String
     def isTermName: Boolean