* Parse JSDoc ...T and T= only at top-level JSDoc
...T and T= should only be legal at the top level of a type, and only in
JSDoc, since at least T= is ambiguous elsewhere. This PR changes parsing
to make that happen. The resulting parse tree is now simpler, allowing
me to get rid of some code I had to add in the checker.
* Extract JSDoc type parsing into its own function
* PR comments:return from parseJSDocType
