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/valueClasses/identityComparisonWithValueClasses.fir.kt

@@ -14,11 +14,11 @@ value class Bar(val y: String)
 fun test(f1: Foo, f2: Foo, b1: Bar, fn1: Foo?, fn2: Foo?) {
     val a1 = f1 === f2 || f1 !== f2
     val a2 = f1 === f1
-    val a3 = f1 === b1 || f1 !== b1
+    val a3 = <!EQUALITY_NOT_APPLICABLE_WARNING!>f1 === b1<!> || <!EQUALITY_NOT_APPLICABLE_WARNING!>f1 !== b1<!>
 
     val c1 = fn1 === fn2 || fn1 !== fn2
     val c2 = f1 === fn1 || f1 !== fn1
-    val c3 = b1 === fn1 || b1 !== fn1
+    val c3 = <!EQUALITY_NOT_APPLICABLE_WARNING!>b1 === fn1<!> || <!EQUALITY_NOT_APPLICABLE_WARNING!>b1 !== fn1<!>
 
     val any = Any()