FIR checker: differentiate UNSAFE_CALL from INAPPLICABLE_CANDIDATE

To do so, inside the root cause of inapplicable candidate errors,
we will record expected/actual type of receiver, if any.
That will help identifying inapplicable calls on nullable receiver.

compiler/testData/diagnostics/tests/when/withSubjectVariable/capturingInInitializer.fir.kt

@@ -27,11 +27,11 @@ fun testUnsafeCaptureVarInInitializer() {
 
     val s = when (val y = run { x = null; 32 }) {
         0 -> {
-            x.<!INAPPLICABLE_CANDIDATE!>inc<!>() // NB smart cast should be impossible
+            x.<!UNSAFE_CALL!>inc<!>() // NB smart cast should be impossible
             "0"
         }
         else -> "!= 0"
     }
 
-    x.<!INAPPLICABLE_CANDIDATE!>inc<!>() // NB smart cast should be impossible
+    x.<!UNSAFE_CALL!>inc<!>() // NB smart cast should be impossible
 }

compiler/testData/diagnostics/tests/when/withSubjectVariable/jumpoutInInitializer.fir.kt

@@ -12,7 +12,7 @@ fun testJumpOutInElvis(x: Int?) {
         x.inc()
     }
 
-    x.<!INAPPLICABLE_CANDIDATE!>inc<!>()
+    x.<!UNSAFE_CALL!>inc<!>()
 }
 
 fun testJumpOutInElvisLikeIf(x: Int?) {
@@ -24,7 +24,7 @@ fun testJumpOutInElvisLikeIf(x: Int?) {
         x.inc()
     }
 
-    x.<!INAPPLICABLE_CANDIDATE!>inc<!>()
+    x.<!UNSAFE_CALL!>inc<!>()
 }
 
 
@@ -36,7 +36,7 @@ fun testJumpOutInIf(x: Int?) {
             0 -> "0"
             else -> "not 0"
         }
-        x.<!INAPPLICABLE_CANDIDATE!>inc<!>()
+        x.<!UNSAFE_CALL!>inc<!>()
     }
 
     x.inc() // Actually, safe, but it's OK if it's error

compiler/testData/diagnostics/tests/when/withSubjectVariable/smartCastOnValueBoundToSubjectVariable.fir.kt

@@ -18,8 +18,8 @@ fun test2(x: String?) {
 
 fun test3(x: String?, y: String?) {
     when (val z = x ?: y!!) {
-        "foo" -> x.<!INAPPLICABLE_CANDIDATE!>length<!>
-        "bar" -> y.<!INAPPLICABLE_CANDIDATE!>length<!>
+        "foo" -> x.<!UNSAFE_CALL!>length<!>
+        "bar" -> y.<!UNSAFE_CALL!>length<!>
         "baz" -> z.length
     }
 }