Fix incorrect name of index signature flag in implementation (#40541)

src/compiler/checker.ts

@@ -13643,7 +13643,7 @@ namespace ts {
                         return undefined;
                     }
                     const shouldIncludeUndefined =
-                        compilerOptions.noUncheckedIndexSignatures &&
+                        compilerOptions.noUncheckedIndexedAccess &&
                         (accessFlags & (AccessFlags.Writing | AccessFlags.ExpressionPosition)) === AccessFlags.ExpressionPosition;
                     if (accessNode && !isTypeAssignableToKind(indexType, TypeFlags.String | TypeFlags.Number)) {
                         const indexNode = getIndexNodeForAccessExpression(accessNode);

src/compiler/types.ts

@@ -5783,7 +5783,7 @@ namespace ts {
         assumeChangesOnlyAffectDirectDependencies?: boolean;
         noLib?: boolean;
         noResolve?: boolean;
-        noUncheckedIndexSignatures?: boolean;
+        noUncheckedIndexedAccess?: boolean;
         out?: string;
         outDir?: string;
         outFile?: string;