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/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-decrement-expression/p-4/neg/1.1.fir.kt

@@ -15,7 +15,7 @@ class A() {
 
 fun case1() {
     var b: Case1? = Case1()
-    <!INAPPLICABLE_CANDIDATE!>--<!>b?.a
+    <!UNSAFE_CALL!>--<!>b?.a
 }
 
 

compiler/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-increment-expression/p-4/neg/1.1.fir.kt

@@ -15,7 +15,7 @@ class A() {
 
 fun case1() {
     var b: Case1? = Case1()
-    <!INAPPLICABLE_CANDIDATE!>++<!>b?.a
+    <!UNSAFE_CALL!>++<!>b?.a
 }