KT-30419 Box inline classes in return types of covariant overrides

Use boxed version of an inline class in return type position for
covariant and generic-specialized overrides.

Also fixes KT-35234 and KT-31585.

compiler/tests/org/jetbrains/kotlin/codegen/BridgeTest.kt

@@ -52,7 +52,7 @@ class BridgeTest : TestCase() {
         return Fun(text)
     }
 
-    private fun bridge(from: Fun, to: Fun): Bridge<Meth> = Bridge(Meth(from), Meth(to))
+    private fun bridge(from: Fun, to: Fun): Bridge<Meth, Fun> = Bridge(Meth(from), Meth(to), emptySet())
 
     /**
      * Constructs a graph out of the given pairs of vertices. First vertex should be a function in the derived class,
@@ -125,13 +125,17 @@ class BridgeTest : TestCase() {
         }
     }
 
-    private fun doTest(function: Fun, expectedBridges: Set<Bridge<Meth>>) {
+    private fun doTest(function: Fun, expectedBridges: Set<Bridge<Meth, Fun>>) {
         val actualBridges = generateBridges(function, ::Meth)
-        assert(actualBridges.firstOrNull { it.from == it.to } == null) {
+        assert(actualBridges.all { it.from != it.to }) {
             "A bridge invoking itself was generated, which makes no sense, since it will result in StackOverflowError" +
                     " once called: $actualBridges"
         }
-        assertEquals(expectedBridges, actualBridges, "Expected and actual bridge sets differ for function $function")
+        assertEquals(
+            expectedBridges.map { it.from to it.to },
+            actualBridges.map { it.from to it.to },
+            "Expected and actual bridge sets differ for function $function"
+        )
     }
 
     // ------------------------------------------------------------------------------------------------------------------