Add semicolon to unused class member list

Turns out SemicolonClassElement is a specific kind for semicolons inside
a class. Having one of them with --noUnusedLocals on would crash the
compiler after the assert added in #21013.

src/compiler/checker.ts

@@ -21030,6 +21030,7 @@ namespace ts {
                             }
                             break;
                         case SyntaxKind.IndexSignature:
+                        case SyntaxKind.SemicolonClassElement:
                             // Can't be private
                             break;
                         default: