[backport] Make the CharSequence wrappers in Predef non-implicit, for JDK 15

In JDK 15 CharSequence has an isEmpty method with a default
implementation, which clashes with our Array[Char]#isEmpty,
IndexedSeq[Char]#isEmpty, as well as our StringBuilder and reflect's
Name.

Backport of scala/scala#9292 from 2.13.x to 2.12.x

Author: dwijnand

library/src/scala/Predef.scala

@@ -97,9 +97,9 @@ import scala.io.StdIn
  * @groupprio implicit-classes-any 70
  * @groupdesc implicit-classes-any These implicit classes add useful extension methods to every type.
  *
- * @groupname implicit-classes-char CharSequence Conversions
- * @groupprio implicit-classes-char 80
- * @groupdesc implicit-classes-char These implicit classes add CharSequence methods to Array[Char] and IndexedSeq[Char] instances.
+ * @groupname char-sequence-wrappers CharSequence Wrappers
+ * @groupprio char-sequence-wrappers 80
+ * @groupdesc char-sequence-wrappers Wrappers that implements CharSequence and were implicit classes.
  *
  * @groupname conversions-java-to-anyval Java to Scala
  * @groupprio conversions-java-to-anyval 90
@@ -349,22 +349,28 @@ object Predef extends LowPriorityImplicits with DeprecatedPredef {
   // and `@deprecatedName(Symbol("<none>"), "2.12.0")` crashes scalac with
   //   scala.reflect.internal.Symbols$CyclicReference: illegal cyclic reference involving object Symbol
   // in run/repl-no-imports-no-predef-power.scala.
-  /** @group implicit-classes-char */
-  implicit final class SeqCharSequence(@deprecated("will be made private", "2.12.0") @deprecatedName(null, "2.12.0") val __sequenceOfChars: scala.collection.IndexedSeq[Char]) extends CharSequence {
+  /** @group char-sequence-wrappers */
+  final class SeqCharSequence(@deprecated("will be made private", "2.12.0") @deprecatedName(null, "2.12.0") val __sequenceOfChars: scala.collection.IndexedSeq[Char]) extends CharSequence {
     def length: Int                                     = __sequenceOfChars.length
     def charAt(index: Int): Char                        = __sequenceOfChars(index)
     def subSequence(start: Int, end: Int): CharSequence = new SeqCharSequence(__sequenceOfChars.slice(start, end))
     override def toString                               = __sequenceOfChars mkString ""
   }
 
-  /** @group implicit-classes-char */
-  implicit final class ArrayCharSequence(@deprecated("will be made private", "2.12.0") @deprecatedName(null, "2.12.0") val __arrayOfChars: Array[Char]) extends CharSequence {
+  /** @group char-sequence-wrappers */
+  def SeqCharSequence(sequenceOfChars: scala.collection.IndexedSeq[Char]): SeqCharSequence = new SeqCharSequence(sequenceOfChars)
+
+  /** @group char-sequence-wrappers */
+  final class ArrayCharSequence(@deprecated("will be made private", "2.12.0") @deprecatedName(null, "2.12.0") val __arrayOfChars: Array[Char]) extends CharSequence {
     def length: Int                                     = __arrayOfChars.length
     def charAt(index: Int): Char                        = __arrayOfChars(index)
     def subSequence(start: Int, end: Int): CharSequence = new runtime.ArrayCharSequence(__arrayOfChars, start, end)
     override def toString                               = __arrayOfChars mkString ""
   }
 
+  /** @group char-sequence-wrappers */
+  def ArrayCharSequence(arrayOfChars: Array[Char]): ArrayCharSequence = new ArrayCharSequence(arrayOfChars)
+
   implicit val StringCanBuildFrom: CanBuildFrom[String, Char, String] = new CanBuildFrom[String, Char, String] {
     def apply(from: String) = apply()
     def apply()             = mutable.StringBuilder.newBuilder

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

@@ -447,6 +447,15 @@ final class StringBuilder(private val underlying: JavaStringBuilder)
    *  @return  the string assembled by this StringBuilder
    */
   def result(): String = toString
+
+
+  /** Tests whether this builder is empty.
+   *
+   *  This method is required for JDK15+ compatibility
+   *
+   *  @return  `true` if this builder contains nothing, `false` otherwise.
+   */
+  override def isEmpty: Boolean = underlying.length() == 0
 }
 
 object StringBuilder {