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/ExtensionFunctions.fir.kt

@@ -64,7 +64,7 @@ import outer.*
 
             command.foo
 
-            command.<!INAPPLICABLE_CANDIDATE!>equals<!>(null)
+            command.<!UNSAFE_CALL!>equals<!>(null)
             command?.equals(null)
             command.equals1(null)
             command?.equals1(null)

compiler/testData/diagnostics/tests/extensions/kt1875.fir.kt

@@ -9,13 +9,13 @@ interface T {
 }
 
 fun test(t: T) {
-    t.<!INAPPLICABLE_CANDIDATE!>f<!>(1) //unsafe call error
+    t.<!UNSAFE_CALL!>f<!>(1) //unsafe call error
     t.f?.invoke(1)
 }
 
 fun test1(t: T?) {
     t.<!UNRESOLVED_REFERENCE!>f<!>(1) // todo resolve f as value and report UNSAFE_CALL
-    t?.<!INAPPLICABLE_CANDIDATE!>f<!>(1)
-    t.<!INAPPLICABLE_CANDIDATE!>f<!>?.invoke(1)
+    t?.<!UNSAFE_CALL!>f<!>(1)
+    t.<!UNSAFE_CALL!>f<!>?.invoke(1)
     t?.f?.invoke(1)
 }

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<!>()
 }