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/sealed/TreeWhenFunctionalNoIs.fir.kt

@@ -11,7 +11,7 @@ sealed class Tree {
 
     fun maxIsClass(): Int = <!NO_ELSE_IN_WHEN!>when<!>(this) {
         Empty -> -1
-        <!NO_COMPANION_OBJECT!>Leaf<!>  -> 0
+        <!INCOMPATIBLE_TYPES, NO_COMPANION_OBJECT!>Leaf<!>  -> 0
         is Node  -> this.left.max()
     }