[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/unitConversion/chainedFunSuspendUnitConversion.fir.kt

@@ -15,4 +15,4 @@ fun test(f: () -> String, s: SubInt) {
     <!INAPPLICABLE_CANDIDATE!>foo<!>(f)
     <!INAPPLICABLE_CANDIDATE!>foo<!>(s)
     foo(::bar)
-}
+}