[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/kt2216.fir.kt

@@ -16,7 +16,7 @@ fun foo() {
     val y: Int? = 0
     val z: Int? = 0
     <!INAPPLICABLE_CANDIDATE!>bar<!>(if (y != null) y else z, y)
-    y <!INAPPLICABLE_CANDIDATE!>+<!> 2
+    y <!NONE_APPLICABLE!>+<!> 2
     <!INAPPLICABLE_CANDIDATE!>baz<!>(y, y, if (y == null) return else y, y)
     baz(y, z!!, z, y)
 }