Implement common Throwable.addSuppressed/suppressed extensions

Instead of `suppressed` Array we provide `suppressedExceptions` List

#KT-23737

libraries/stdlib/jdk7/testNoJdk7/ExceptionTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -15,9 +15,11 @@ class ExceptionTest {
         val e1 = Throwable()
         val e2 = Exception("Suppressed")
 
+        assertTrue(e1.suppressedExceptions.isEmpty())
         e1.addSuppressedExtension(e2)
 
         assertSame(e2, e1.suppressed.singleOrNull())
+        assertSame(e2, e1.suppressedExceptions.singleOrNull())
     }
 
 }