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/resolve/invoke/errors/unsafeCallWithInvoke.fir.kt

@@ -4,7 +4,7 @@
 operator fun String.invoke(i: Int) {}
 
 fun foo(s: String?) {
-    <!INAPPLICABLE_CANDIDATE!>s<!>(1)
+    <!UNSAFE_CALL!>s<!>(1)
 
-    <!INAPPLICABLE_CANDIDATE!>(s ?: null)(1)<!>
+    <!UNSAFE_CALL!>(s ?: null)(1)<!>
 }