prevent value assignments to @typedef names

[blickly]

This issue was imported from Closure Compiler's previous home at http://closure-compiler.googlecode.com

The original discussion is archived at:
http://blickly.github.io/closure-compiler-issues/#1005

[Dominator008]

We should probably warn on:

/** @typedef {string} */
var x = 'blah';

[dimvar]

NTI warns, I doubt that we'll fix for the old type checker.

[blickly]

Seems like something that could be checked completely syntactically (independently of the type checker)