Incorrect type declarations for a constant inside a namespace merged with an enum

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### 🔎 Search Terms

"enum", "namespace", "declaration", "d.ts", "const"

### 🕗 Version & Regression Information

- This changed between versions 3.1.0-dev.20180906 and 3.1.0-dev.20180907

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/KYOwrgtgBAYg9nKBvAUFdUBGBDATigXxRGwmAGcAHbAY2FgWTQ2AA9K5cAXKGuEcjxwAvKAF4GcAHQ5cAbkIogA

### 💻 Code

```ts
enum Foo {
bar
}
namespace Foo {
export const baz = Foo.bar;
}
```

### 🙁 Actual behavior

Typescript generates invalid d.ts for the enum value assigned to the constant (in all versions starting 3.1.0-dev.20180907)
```ts
declare enum Foo {
bar = 0
}
declare namespace Foo {
const baz = bar; // <-- unknown `bar` identifier, `baz` type is `any`
}
```
In versions before 3.1.0-dev.20180906 the constant is declared as `const baz: Foo;`

### 🙂 Expected behavior

Typescript generates a valid declaration of the constant
```ts
declare enum Foo {
bar = 0
}
declare namespace Foo {
const baz = Foo.bar;
}
```
or
```ts
declare enum Foo {
bar = 0
}
declare namespace Foo {
const baz: Foo;
}
```

### Additional information about the issue

_No response_