Check return types of generator functions.

[brandonbloom]

Fixes #18726

[brandonbloom]

Some notes:

• This doesn't address the done property of IteratorResult, nor does it address the return value of yield expressions.
• Making the return method have a return type of IteratorResult<any> is awkward, but is actually a more accurate type. If you call next on an iterator manually, you still get an IteratorResult<T> where value is undefined if done is true... unless you return something from a generator function, then you can get anything there.
• In theory, this only relaxes the type of iterators, so no code should be broken, but I'm sure you guys will validate that.
• I tried to run generate-diagnostics, but the changes to the .json file didn't seem to get incorporated in to the .js output. Didn't dig in yet, was hoping it's an easy fix with a hint.
• I'd be happy to help with code to fix Boolean literal types and return type propagation for generators #2983 or yield expressions should be able to acquire the type of generator return types #10509 if provided some guidance. I'm working on an interpreter that abuses co-routines, so these issues affect the bulk of my work.

[typescript-bot]

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