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/smartCasts/publicVals/customGetter.fir.kt

@@ -5,7 +5,7 @@ public class X {
     public fun fn(): Int {
         if (y != null)
             // With non-default getter smartcast is not possible
-            return y.<!INAPPLICABLE_CANDIDATE!>length<!>
+            return y.<!UNSAFE_CALL!>length<!>
         else
             return 0
     }

compiler/testData/diagnostics/tests/smartCasts/publicVals/open.fir.kt

@@ -7,5 +7,5 @@ infix fun Int.bar(i: Int) = i
 fun test() {
     val p = A()
     // For open value properties, smart casts should not work
-    if (p.foo is Int) p.foo <!INAPPLICABLE_CANDIDATE!>bar<!> 11
+    if (p.foo is Int) p.foo <!UNSAFE_CALL!>bar<!> 11
 }

compiler/testData/diagnostics/tests/smartCasts/publicVals/var.fir.kt

@@ -4,10 +4,10 @@ public class X {
     public fun fn(): Int {
         if (x != null)
             // Smartcast is not possible for variable properties
-            return x.<!INAPPLICABLE_CANDIDATE!>length<!>
+            return x.<!UNSAFE_CALL!>length<!>
         else if (y != null)
             // Even if they are private
-            return y.<!INAPPLICABLE_CANDIDATE!>length<!>
+            return y.<!UNSAFE_CALL!>length<!>
         else
             return 0
     }