Allow compile-time computable expressions to initialize ambient `const`s

To support #6678, allow `const` declarations in ambient contexts to have compile-time computable initializers:

``` ts
// foo.d.ts
const x = 'hello'; // OK
const y = 'hello ' + 'world' // OK
const z = 'hello'.substr(3); // Error

const a = 10; // OK
const b = 10 + 10; // OK
// Rest of rules follow from 'const enum' rules

const m = true; // OK
const n = false; // OK
```
