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/testsWithStdLib/resolve/samOverloadsWithKtFunction.fir.kt

@@ -16,5 +16,5 @@ public class Foo {
 
 // FILE: 1.kt
 fun bar() {
-    Foo().<!AMBIGUITY!>test<!> {} <!INAPPLICABLE_CANDIDATE!>checkType<!> { <!INAPPLICABLE_CANDIDATE!>_<!><String>() }
+    Foo().<!AMBIGUITY!>test<!> {} <!UNSAFE_CALL!>checkType<!> { <!INAPPLICABLE_CANDIDATE!>_<!><String>() }
 }