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/when/When.fir.kt

@@ -23,7 +23,7 @@ fun foo() : Int {
       <!USELESS_IS_CHECK!>!is Int<!> -> 1
       <!USELESS_IS_CHECK!>is Any?<!> -> 1
       <!USELESS_IS_CHECK!>is Any<!> -> 1
-      s -> 1
+      <!INCOMPATIBLE_TYPES!>s<!> -> 1
       1 -> 1
       1 <!OVERLOAD_RESOLUTION_AMBIGUITY!>+<!> <!UNRESOLVED_REFERENCE!>a<!> -> 1
       in 1..<!UNRESOLVED_REFERENCE!>a<!> -> 1
@@ -41,8 +41,8 @@ fun test() {
   val s = "";
 
   when (x) {
-    s -> 1
-    "" -> 1
+    <!INCOMPATIBLE_TYPES!>s<!> -> 1
+    <!INCOMPATIBLE_TYPES!>""<!> -> 1
     x -> 1
     1 -> 1
   }