Allow delegating val properties to out-projected MutableMap

Change generic signature of MutableMap.getValue, use 'out @Exact V' and `V1: V` types instead of single `in V`.
Fix affected IR generation tests.

#KT-18789 Fixed

libraries/stdlib/test/properties/delegation/MapAccessorsTest.kt

@@ -5,6 +5,8 @@ import kotlin.test.*
 class ValByMapExtensionsTest {
     val map: Map<String, String> = hashMapOf("a" to "all", "b" to "bar", "c" to "code")
     val genericMap = mapOf<String, Any?>("i" to 1, "x" to 1.0)
+    val mmapOut: MutableMap<String, out String> = mutableMapOf("g" to "out", "g1" to "in")
+    val genericMmapOut: MutableMap<String, out Any?> = mmapOut
 
     val a by map
     val b: String by map
@@ -12,6 +14,8 @@ class ValByMapExtensionsTest {
     val d: String? by map
     val e: String by map.withDefault { "default" }
     val f: String? by map.withDefault { null }
+    val g: String by mmapOut
+    val g1: String by genericMmapOut
     // val n: Int by map // prohibited by type system
     val i: Int by genericMap
     val x: Double by genericMap
@@ -23,6 +27,8 @@ class ValByMapExtensionsTest {
         assertEquals("code", c)
         assertEquals("default", e)
         assertEquals(null, f)
+        assertEquals("out", g)
+        assertEquals("in", g1)
         assertEquals(1, i)
         assertEquals(1.0, x)
         assertFailsWith<NoSuchElementException> { d }