New J2K: separate nullability inference from common one & nullability bug fixes

It will be needed for structure mutability inference

#KT-21467 fixed
#KT-32609 fixed
#KT-32572 fixed
#KT-24677 fixed

nj2k/testData/inference/nullability/forcedNullability.kt

@@ -0,0 +1,9 @@
+fun b(m: /*T2@*/Map</*T0@*/Int, /*T1@*/String>): /*T3@*/String? {
+    return m/*T2@Map<T0@Int, T1@String>*/.get(42/*LIT*/)/*T1@String!!U*/
+}
+
+//T0 := T0 due to 'RECEIVER_PARAMETER'
+//T1 <: T1 due to 'RECEIVER_PARAMETER'
+//LOWER <: T0 due to 'PARAMETER'
+//T2 := LOWER due to 'USE_AS_RECEIVER'
+//UPPER <: T3 due to 'RETURN'