Do not mutate Matcher in MatchResult.next()

Instead of mutating the matcher create a new one when `next()` is called.
This allows getting named groups from that matcher later.

Add lookbehind in matchSequence test to ensure this change doesn't alter
the existing behavior.

Also fixes #KT-20865

libraries/stdlib/jdk8/test/text/RegexTest.kt

@@ -21,22 +21,28 @@ import kotlin.test.*
 
 class RegexTest {
     @Test fun namedGroups() {
-        val input = "1a 2b 3c"
-        val regex = "(?<num>\\d)(?<liter>\\w)".toRegex()
+        val input = "1a 2b 3"
+        val regex = "(?<num>\\d)(?<liter>\\w)?".toRegex()
 
         val matches = regex.findAll(input).toList()
         assertTrue(matches.all { it.groups.size == 3 })
-        val m1 = matches[0]
+        val (m1, m2, m3) = matches
 
         assertEquals("1", m1.groups["num"]?.value)
         assertEquals(0..0, m1.groups["num"]?.range)
         assertEquals("a", m1.groups["liter"]?.value)
         assertEquals(1..1, m1.groups["liter"]?.range)
 
-        val m2 = matches[1]
         assertEquals("2", m2.groups["num"]?.value)
         assertEquals(3..3, m2.groups["num"]?.range)
         assertEquals("b", m2.groups["liter"]?.value)
         assertEquals(4..4, m2.groups["liter"]?.range)
+
+        assertEquals("3", m3.groups["num"]?.value)
+        assertNull(m3.groups["liter"])
+
+        assertFailsWith<IllegalArgumentException> { m2.groups["unknown_group"] }.let { e ->
+            assertTrue("unknown_group" in e.message!!)
+        }
     }
 }