[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/testsWithStdLib/smartcasts/forEachUnsafe.fir.kt

@@ -3,7 +3,7 @@
 fun indexOfMax(a: IntArray): Int? {
     var maxI: Int? = 0
     a.forEachIndexed { i, value ->
-        if (value <!AMBIGUITY!>>=<!> <!INAPPLICABLE_CANDIDATE!>a[maxI]<!>) {
+        if (value >= <!INAPPLICABLE_CANDIDATE!>a[maxI]<!>) {
             maxI = i
         }
         else if (value < 0) {