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/statements/assignments/p-2/neg/1.2.fir.kt

@@ -8,7 +8,7 @@
  */
 fun case1() {
     val x : Case1? = Case1()
-    x.<!INAPPLICABLE_CANDIDATE!>x<!> = "0"
+    x.x = "0"
     x?.x = "0"
     <!VARIABLE_EXPECTED!>x::x<!> = TODO()
 }
@@ -24,7 +24,7 @@ class Case1{
  */
 fun case2() {
     val x : Case2? = Case2(null)
-    x.<!INAPPLICABLE_CANDIDATE!>x<!> = "0"
+    x.x = "0"
     x?.x = "0"
     <!VARIABLE_EXPECTED!>x::x<!> = TODO()
 }
@@ -37,7 +37,7 @@ class Case2(val x: Any?) {}
  */
 fun case3() {
     val x : Case3? = Case3()
-    x.<!INAPPLICABLE_CANDIDATE!>x<!> = "0"
+    x.x = "0"
     x?.x = "0"
     <!VARIABLE_EXPECTED!>x::x<!> = TODO()
 }