Consider adding @checkReturnValue

Some context: https://github.com/google/closure-compiler/issues/1745 reported that `Object.create()` was not dead code eliminated. My proposed fix in https://github.com/google/closure-compiler/pull/1746 was pointed out by @SLaks to be unsafe because of cases like:

``` js
document.getElementById('foo').innerHTML = 'blah';
```

`document.getElementById` is also marked with `@nosideeffects`, but since it returns a shared mutable object, the assignment cannot be removed.

Another example: https://github.com/google/closure-compiler/pull/1281 removed `@nosideeffects` from `JSON.parse` and `JSON.stringify` because they had both `@throws` and `@nosideeffects`, which is contradictory. After removing `@nosideeffects`, some calls to them will not be removed.

These examples suggest that the definition of `@nosideeffects` is not clear enough. Currently, most of its use cases can be replaced by a `@checkReturnValue` annotation which can adopt similar semantics as the one in [Error Prone](http://errorprone.info/bugpattern/CheckReturnValue). We will need something like a `@returnValueNotShared` annotation for https://github.com/google/closure-compiler/issues/1746. We can also add a `@pure` annotation which can be recognized by `PureFunctionIdentifier`.

IMO we should also allow these new annotations in both externs and code.