Fix type inference issues for 'if' and 'when'.

Use 'expectedType' (when present) as an explicit type argument for a special construct call.
Unfortunately, this approach can't be used for elvis due to other elvis-related inference hacks.
Fixes KT-10807, KT-10811.
This also affects KT-6189: now we can infer proper type for 'if'.

If type inference for special call failed, and we found no type errors in sub-expressions,
report TYPE_INFERENCE_FAILED_ON_SPECIAL_CONSTRUCT error.
This (and the hack above) fixes KT-10809: code no longer compiles.

core/descriptors/src/org/jetbrains/kotlin/builtins/KotlinBuiltIns.java

@@ -1097,6 +1097,10 @@ public abstract class KotlinBuiltIns {
         return isNotNullConstructedFromGivenClass(type, FQ_NAMES.unit);
     }
 
+    public static boolean isUnitOrNullableUnit(@NotNull KotlinType type) {
+        return isConstructedFromGivenClass(type, FQ_NAMES.unit);
+    }
+
     public boolean isBooleanOrSubtype(@NotNull KotlinType type) {
         return KotlinTypeChecker.DEFAULT.isSubtypeOf(type, getBooleanType());
     }