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/extensions/throwOutCandidatesByReceiver.fir.kt

@@ -8,7 +8,7 @@ fun <R> List<R>.a() {}
 
 fun test1(i: Int?) {
     1.<!INAPPLICABLE_CANDIDATE!>a<!>()
-    i.<!INAPPLICABLE_CANDIDATE!>a<!>()
+    i.<!UNSAFE_CALL!>a<!>()
 }
 
 fun <R> test2(c: Collection<R>) {
@@ -19,7 +19,7 @@ fun Int.foo() {}
 
 fun test3(s: String?) {
     "".<!INAPPLICABLE_CANDIDATE!>foo<!>()
-    s.<!INAPPLICABLE_CANDIDATE!>foo<!>()
+    s.<!UNSAFE_CALL!>foo<!>()
     "".<!INAPPLICABLE_CANDIDATE!>foo<!>(1)
     s.<!INAPPLICABLE_CANDIDATE!>foo<!>("a")
 }
@@ -52,5 +52,5 @@ fun test7(l: List<String?>) {
 }
 
 fun test8(l: List<Any>?) {
-    l.<!INAPPLICABLE_CANDIDATE!>b<!>()
+    l.<!UNSAFE_CALL!>b<!>()
 }