I'm surprised we haven't seen more of this; I suspect it's because the
mixed `module.exports=` + `export.foo=` pattern isn't that common.
However, it'll happen any time that the exported symbol is unknown;
getCommonJsExportEquals blithely clones unknownSymbol and proceeds to
stick the `exports.foo=` properties onto it.

This causes problems later, because the compiler checks for
unknownSymbol with `===`. The fix is to not stick properties onto a
clone of unknownSymbol. This makes the correct errors appear and removes
the crash.

In the binder, unreachable code mistakenly skips the `bindJSDoc` call in
`bindChildrenWorker`, which sets parent pointers. The fix is to call
`bindJSDoc` in the case of unreachable code as well.

JSDoc types references can often be to values, which can often be
circular in ways that types tied to declarations cannot. I decided to
create a separate property on SymbolLinks rather than reusing
declaredType, although I'm not sure that's strictly required.

The ad-hoc name resolution rule for `exports` forgets to check the
requested meaning. When `getTypeReferenceType` calls`
resolveTypeReferenceName` with `Type` only in order to give an error
when the program uses a value like a type, it is incorrectly able to
resolve `exports` instead of producing an error. Then this incorrect
symbol gets treated like an alias, which it isn't, causing the assert.

The fix, for now, is to make resolution of `exports` check the requested
meaning so that it only resolves when `Value` is requested. This makes
the above code an error ("Cannot use the namespace 'exports' as a
type."), but I think this is fine for a bug fix. We can decide later if
`exports` should behave like other expandos and be a legal type
reference.

Note that the name actually does resolve correctly, so JS users will get
the desired completions. They'll just have an error to suppress if they
have checkJs on.

`@constructor` put on anything incorrectly makes it a JS constructor. This
is a problem for actual constructors, because getJSClassType doesn't
work on actual classes. The fix is to make isJSConstructor require that
its declaration is a function.

The check for prototype assignment on constructor functions assumes
that the prototype property, if present, comes from an assignment
declaration, such as:

```js
SomeClass.prototype = { /* methods go here */ }
```

In this case, however, when class SomeClass and var SomeClass merge
(because this is allowed), prototype is the synthetic property from
class SomeClass, which has no valueDeclaration.

The fix is to check that prototype has a valueDeclaration before
checking whether the valueDeclaration is in fact a prototype-assignment
declaration.