Make Throwable.addSuppressed extension work without kotlin-stdlib-jdk7*

*given that it runs on JDK7 and higher. The addSuppressed member
is called with reflection when it's available.
kotlin-stdlib-jdk7 extension still overrides that and calls
addSuppressed member statically as before.

#KT-30560 Fixed

libraries/stdlib/jdk7/testNoJdk7/ExceptionTest.kt

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package test.exceptions
+
+import kotlin.addSuppressed as addSuppressedExtension
+import kotlin.test.*
+
+class ExceptionTest {
+
+    @Test
+    fun addSuppressedWorksWithoutJdk7Extensions() {
+        val e1 = Throwable()
+        val e2 = Exception("Suppressed")
+
+        e1.addSuppressedExtension(e2)
+
+        assertSame(e2, e1.suppressed.singleOrNull())
+    }
+
+}