fix: const enums + isolatedModules emit invalid code

In `isolatedModules` mode, the compiler does not inline const enums,
but also decides not to `import` them, leaving invalid code that
throws a `ReferenceError` at runtime.

This code:

```
import { SomeEnum } from './bar';
sink(SomeEnum.VALUE);
```

..should compile to either:

```
var { SomeEnum } = require('./bar');
sink(SomeEnum.VALUE);
```

..or (with const enum inlining):

```
sink(1 /* VALUE */);
```

..but actually compiles to:
```
sink(SomeEnum.VALUE);
```

..with no imports, which throws a ReferenceError at runtime.

---

The compiler has already realised that the symbol is a referenced const
enum, it just doesn't use this information when it comes to deciding
whether to emit an import. This commit additionally checks that
information, if we are compiling in isolatedModules mode.

---

In my opinion, this is not the correct fix, but it is the simplest. In
`isolatedModules` mode, `const enum` should probably be a compile error
(because there are no benefits and the complexity is high, and,
apparently, buggy). If not, the compiler should not be checking whether
something is a const enum, and should just be treating it as a regular
enum, and checking as if it was?

Fixes microsoft#40499.

Author: FauxFaux

src/compiler/checker.ts

@@ -38211,7 +38211,8 @@ namespace ts {
         function isReferencedAliasDeclaration(node: Node, checkChildren?: boolean): boolean {
             if (isAliasSymbolDeclaration(node)) {
                 const symbol = getSymbolOfNode(node);
-                if (symbol && getSymbolLinks(symbol).referenced) {
+                const links = symbol && getSymbolLinks(symbol);
+                if (links?.referenced || (compilerOptions.isolatedModules && links?.constEnumReferenced)) {
                     return true;
                 }
                 const target = getSymbolLinks(symbol!).target; // TODO: GH#18217

tests/baselines/reference/isolatedModulesImportConstEnum.js

@@ -21,4 +21,5 @@ var Foo;
 //// [file1.js]
 "use strict";
 exports.__esModule = true;
+var file2_1 = require("./file2");
 console.log(file2_1.Foo.BAR);