Add helper functions to simplify getCompletionEntryDisplayNameForSymbol (#20552)

src/compiler/checker.ts

@@ -8128,7 +8128,7 @@ namespace ts {
         }
 
         function getLiteralTypeFromPropertyName(prop: Symbol) {
-            return getDeclarationModifierFlagsFromSymbol(prop) & ModifierFlags.NonPublicAccessibilityModifier || startsWith(prop.escapedName as string, "__@") ?
+            return getDeclarationModifierFlagsFromSymbol(prop) & ModifierFlags.NonPublicAccessibilityModifier || isKnownSymbol(prop) ?
                 neverType :
                 getLiteralType(symbolName(prop));
         }

src/compiler/utilities.ts

@@ -2119,6 +2119,10 @@ namespace ts {
         return "__@" + symbolName as __String;
     }
 
+    export function isKnownSymbol(symbol: Symbol): boolean {
+        return startsWith(symbol.escapedName as string, "__@");
+    }
+
     /**
      * Includes the word "Symbol" with unicode escapes
      */