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/IdentityComparisonWithPrimitives.fir.kt

@@ -32,7 +32,7 @@ val test_dd = d === d || d !== d
 val test_cc = c === c || c !== c
 
 // Identity for primitive values of different types (no extra error)
-val test_zb = z === b || z !== b
+val test_zb = <!EQUALITY_NOT_APPLICABLE!>z === b<!> || <!EQUALITY_NOT_APPLICABLE!>z !== b<!>
 
 // Primitive vs nullable
 val test_znz = z === nz || nz === z || z !== nz || nz !== z