[FIR] Implement NULL_FOR_NONNULL_TYPE diagnostics, fix tests

compiler/testData/diagnostics/tests/generics/multipleBoundsMemberScope/properties.fir.kt

@@ -1,19 +0,0 @@
-// !CHECK_TYPE
-
-interface A {
-    val foo: Any?
-}
-
-interface C: A {
-    override val foo: String?
-}
-interface B: A {
-    override var foo: String
-}
-
-fun <T> test(a: T) where T : B, T : C {
-    a.foo = ""
-    a.foo = null
-
-    a.foo.checkType { _<String>() }
-}

compiler/testData/diagnostics/tests/generics/multipleBoundsMemberScope/properties.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !CHECK_TYPE
 
 interface A {