FIR: add equality call checker

Added checker for FirEqualityOperatorCall. It's surfaced as one of the
following diagnostics depending on the PSI structure and types under
comparison:

* INCOMPATIBLE_TYPES(_WARNING)
* EQUALITY_NOT_APPLICABLE(_WARNING)
* INCOMPATIBLE_ENUM_COMPARISON_ERROR

Comparing with FE1.0, the current implementation is more conservative
and only highlights error if the types are known to follow certain
contracts with `equals` method. Otherwise, the checker reports warnings
instead.

However, the current checker is more strict in the following situations:
1. it now rejects incompatible enum types like `Enum<E1>` and
  `Enum<E2>`, which was previously accepted
2. it now rejects incompatible class types like `Class<String>` and
  `Class<Int>`, which was previously accepted
3. the check now takes smart cast into consideration, so
  `if (x is String) x == 3` is now rejected

compiler/testData/diagnostics/tests/classLiteral/smartCast.fir.kt

@@ -5,7 +5,7 @@ import kotlin.reflect.KClass
 class Foo {
     override fun equals(other: Any?): Boolean {
         if (this === other) return true
-        if (other === null || other::class != this::class) return false
+        if (other === null || <!EQUALITY_NOT_APPLICABLE!>other::class != this::class<!>) return false
 
         return true
     }