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/tests-spec/testData/diagnostics/notLinked/dfa/pos/6.fir.kt

@@ -577,7 +577,7 @@ fun case_29(x: Boolean) {
     if (false || false || false || false || y !== v) {
         val t = <!DEBUG_INFO_EXPRESSION_TYPE("<anonymous>?")!><!UNSAFE_IMPLICIT_INVOKE_CALL!>y<!>()<!>
 
-        if (z !== t || false) {
+        if (<!EQUALITY_NOT_APPLICABLE_WARNING!>z !== t<!> || false) {
             <!DEBUG_INFO_EXPRESSION_TYPE("<anonymous>?")!>t<!><!UNSAFE_CALL!>.<!>a
             <!DEBUG_INFO_EXPRESSION_TYPE("<anonymous>?")!>t<!><!UNSAFE_CALL!>.<!>equals(null)
             <!DEBUG_INFO_EXPRESSION_TYPE("<anonymous>?")!>t<!>.propT