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/nullabilityAndSmartCasts/QualifiedExpressionNullability.fir.kt

@@ -7,7 +7,7 @@ fun main() {
     val x: Foo? = null
     val y: Foo? = null
     
-    x.<!INAPPLICABLE_CANDIDATE!>foo<!>(y)
+    x.<!UNSAFE_CALL!>foo<!>(y)
     x!!.<!INAPPLICABLE_CANDIDATE!>foo<!>(y)
     x.foo(y!!)
     x!!.foo(y!!)
@@ -16,8 +16,8 @@ fun main() {
     val b: Foo? = null
     val c: Foo? = null
     
-    a.<!INAPPLICABLE_CANDIDATE!>foo<!>(b.<!INAPPLICABLE_CANDIDATE!>foo<!>(c))
-    a!!.foo(b.<!INAPPLICABLE_CANDIDATE!>foo<!>(c))
+    a.<!UNSAFE_CALL!>foo<!>(b.<!UNSAFE_CALL!>foo<!>(c))
+    a!!.foo(b.<!UNSAFE_CALL!>foo<!>(c))
     a.foo(b!!.<!INAPPLICABLE_CANDIDATE!>foo<!>(c))
     a!!.foo(b!!.<!INAPPLICABLE_CANDIDATE!>foo<!>(c))
     a.foo(b.foo(c!!))