[FIR] Improve diagnostic reporting & don't use error symbol for candidate if possible

Also introduce few new diagnostics:
- NONE_APPLICABLE more many inapplicable candidates
- HIDDEN for visible candidates

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/NullableNothingIsExactlyNull.fir.kt

@@ -2,7 +2,7 @@ fun test() {
   val out : Int? = null
   val x : Nothing? = null
   if (out != x)
-    out.<!INAPPLICABLE_CANDIDATE!>plus<!>(1)
+    out.<!NONE_APPLICABLE!>plus<!>(1)
   if (out == x) return
-  out.<!INAPPLICABLE_CANDIDATE!>plus<!>(1)
+  out.<!NONE_APPLICABLE!>plus<!>(1)
 }