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/fir/analysis-tests/testData/resolve/whenElse.kt

@@ -45,12 +45,12 @@ fun case3() {
 
     val flag = ""  //A
     val l1 = <!NO_ELSE_IN_WHEN!>when<!> (flag<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>) {// should be NO_ELSE_IN_WHEN
-        A.A1 -> B() //should be INCOMPATIBLE_TYPES
-        A.A2 -> B() //should be INCOMPATIBLE_TYPES
+        <!INCOMPATIBLE_TYPES!>A.A1<!> -> B() //should be INCOMPATIBLE_TYPES
+        <!INCOMPATIBLE_TYPES!>A.A2<!> -> B() //should be INCOMPATIBLE_TYPES
     }
 
     val l2 = <!NO_ELSE_IN_WHEN!>when<!> (flag) {// should be NO_ELSE_IN_WHEN
-        A.A1 -> B() //should be INCOMPATIBLE_TYPES
-        A.A2 -> B() //should be INCOMPATIBLE_TYPES
+        <!INCOMPATIBLE_TYPES!>A.A1<!> -> B() //should be INCOMPATIBLE_TYPES
+        <!INCOMPATIBLE_TYPES!>A.A2<!> -> B() //should be INCOMPATIBLE_TYPES
     }
 }