Cherry-pick PR microsoft#54317 into release-5.1

Component commits:
18b8b63 Resolve re-exports when looking for tslib helpers

2d702aa Accept baselines

Author: andrewbranch

src/compiler/checker.ts

@@ -46998,7 +46998,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                                 if (requestedExternalEmitHelperNames.has(name)) continue;
                                 requestedExternalEmitHelperNames.add(name);
 
-                                const symbol = getSymbol(helpersModule.exports!, escapeLeadingUnderscores(name), SymbolFlags.Value);
+                                const symbol = getSymbol(getExportsOfModule(helpersModule), escapeLeadingUnderscores(name), SymbolFlags.Value);
                                 if (!symbol) {
                                     error(location, Diagnostics.This_syntax_requires_an_imported_helper_named_1_which_does_not_exist_in_0_Consider_upgrading_your_version_of_0, externalHelpersModuleNameText, name);
                                 }

tests/baselines/reference/tslibReExportHelpers.js

@@ -0,0 +1,39 @@
+//// [tests/cases/compiler/tslibReExportHelpers.ts] ////
+
+//// [index.d.ts]
+export declare function __decorate(...args: any[]): any;
+
+//// [index.d.mts]
+export * from "./index.js";
+
+//// [package.json]
+{
+    "name": "tslib",
+    "version": "1.0.0",
+    "types": "index.d.ts",
+    "exports": {
+        ".": {
+            "types": {
+                "import": "./index.d.mts",
+                "default": "./index.d.ts"
+            }
+        }
+    }
+}
+
+//// [index.mts]
+declare var decorator: any;
+@decorator
+export class Foo {}
+
+
+//// [index.mjs]
+import { __decorate } from "tslib";
+export var Foo = /** @class */ (function () {
+    function Foo() {
+    }
+    Foo = __decorate([
+        decorator
+    ], Foo);
+    return Foo;
+}());

tests/baselines/reference/tslibReExportHelpers.symbols

@@ -0,0 +1,19 @@
+=== /node_modules/tslib/index.d.ts ===
+export declare function __decorate(...args: any[]): any;
+>__decorate : Symbol(__decorate, Decl(index.d.ts, 0, 0))
+>args : Symbol(args, Decl(index.d.ts, 0, 35))
+
+=== /node_modules/tslib/index.d.mts ===
+
+export * from "./index.js";
+
+=== /index.mts ===
+declare var decorator: any;
+>decorator : Symbol(decorator, Decl(index.mts, 0, 11))
+
+@decorator
+>decorator : Symbol(decorator, Decl(index.mts, 0, 11))
+
+export class Foo {}
+>Foo : Symbol(Foo, Decl(index.mts, 0, 27))
+

tests/baselines/reference/tslibReExportHelpers.types

@@ -0,0 +1,19 @@
+=== /node_modules/tslib/index.d.ts ===
+export declare function __decorate(...args: any[]): any;
+>__decorate : (...args: any[]) => any
+>args : any[]
+
+=== /node_modules/tslib/index.d.mts ===
+
+export * from "./index.js";
+
+=== /index.mts ===
+declare var decorator: any;
+>decorator : any
+
+@decorator
+>decorator : any
+
+export class Foo {}
+>Foo : Foo
+

tests/cases/compiler/tslibReExportHelpers.ts

@@ -0,0 +1,29 @@
+// @module: nodenext
+// @experimentalDecorators: true
+// @importHelpers: true
+
+// @Filename: /node_modules/tslib/index.d.ts
+export declare function __decorate(...args: any[]): any;
+
+// @Filename: /node_modules/tslib/index.d.mts
+export * from "./index.js";
+
+// @Filename: /node_modules/tslib/package.json
+{
+    "name": "tslib",
+    "version": "1.0.0",
+    "types": "index.d.ts",
+    "exports": {
+        ".": {
+            "types": {
+                "import": "./index.d.mts",
+                "default": "./index.d.ts"
+            }
+        }
+    }
+}
+
+// @Filename: /index.mts
+declare var decorator: any;
+@decorator
+export class Foo {}