[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/smartCasts/varnotnull/varIntNull.fir.kt

@@ -2,5 +2,5 @@
 fun foo(): Int {
     var i: Int? = 42
     i = null
-    return i <!INAPPLICABLE_CANDIDATE!>+<!> 1
-}
+    return i <!NONE_APPLICABLE!>+<!> 1
+}