[FIR] Forbid erroneous ===-checks

It was decided to forbid such comparisons,
as we know how `===` works. Also, added some more
test cases, just for comparison.

Reusing the proper `canHaveSubtypes()`
from `TypeUtils` prevents a breaking change
in:

- `comparingTripleWithPair.kt`
- `comparisonOfGenericInterfaceWithGenericClass.kt`

But it does lead to warnings
(instead of errors) in
`incompatibleEnumEntryClasses.kt`, which is an
unrelated mistake that will be fixed in the next
commit.

The refactoring in `canHaveSubtypes()` is purely
cosmetic - otherwise reading these conditions is hard
(and they don't fit my screen vertically).

^KT-62646
^KT-65541
^KT-57779

compiler/testData/diagnostics/tests/comparisonOfGenericInterfaceWithGenericClass.kt

@@ -1,7 +1,8 @@
-// FIR_IDENTICAL
 // ISSUE: KT-47884
 
 interface A<X>
 class B<T>
 
 fun foo(a: A<*>, b: B<*>): Boolean = a == b
+
+fun bar(a: A<*>, b: B<*>): Boolean = a === b