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

idea/testData/checker/BinaryCallsOnNullableValues.fir.kt

@@ -13,10 +13,10 @@ fun f(): Unit {
   x == 1
   x != 1
 
-  A() == 1
+  <error descr="[EQUALITY_NOT_APPLICABLE] Operator '==' cannot be applied to 'A' and 'kotlin/Int'">A() == 1</error>
 
-  x === "1"
-  x !== "1"
+  <error descr="[EQUALITY_NOT_APPLICABLE] Operator '===' cannot be applied to 'kotlin/Int' and 'kotlin/String'">x === "1"</error>
+  <error descr="[EQUALITY_NOT_APPLICABLE] Operator '!==' cannot be applied to 'kotlin/Int' and 'kotlin/String'">x !== "1"</error>
 
   x === 1
   x !== 1

idea/testData/checker/When.fir.kt

@@ -7,7 +7,7 @@ fun foo() : Int {
       is String -> 1
       !is Int -> 1
       is Any? -> 1
-      s -> 1
+      <error descr="[INCOMPATIBLE_TYPES] Incompatible types: kotlin/Int and kotlin/String">s</error> -> 1
       1 -> 1
       1 <error descr="[OVERLOAD_RESOLUTION_AMBIGUITY] Overload resolution ambiguity between candidates: [kotlin/Int.plus, kotlin/Int.plus, kotlin/Int.plus, ...]">+</error> <error descr="[UNRESOLVED_REFERENCE] Unresolved reference: a">a</error> -> 1
       in 1..<error descr="[UNRESOLVED_REFERENCE] Unresolved reference: a">a</error> -> 1
@@ -25,8 +25,8 @@ fun test() {
   val s = "";
 
   when (x) {
-    s -> 1
-    "" -> 1
+    <error descr="[INCOMPATIBLE_TYPES] Incompatible types: kotlin/Int and kotlin/String">s</error> -> 1
+    <error descr="[INCOMPATIBLE_TYPES] Incompatible types: kotlin/Int and kotlin/String">""</error> -> 1
     x -> 1
     1 -> 1
     else -> 1