add optional argument to code_lowered to enable generator expansion

[jrevels]

Attempt at fixing #22874. It works, but I'm not sure if this is how it should be implemented.

From Jeff's/Yichao's comments in that issue:

The problem is abstract types, for which we don't invoke generators. In those cases, we'd have to give an error (perhaps until we have fallback code for generated functions)

It should always be safe for @code_lowered though. For code_lowered I guess we can have arguments to control this (just like code_llvm) and pick a reasonable default behavior?

By default, I've disabled generator expansion for code_lowered and enabled it for @code_lowered. I guess the mismatch could be a point of confusion...

I'll add some tests once I get confirmation that this is going in the right direction.

[JeffBezanson]

This will return nothing if mtypes isn't a leaf type.

[JeffBezanson]

I think expand_generated should default to true in both cases.

[jrevels]

I think expand_generated should default to true in both cases.
This will return nothing if mtypes isn't a leaf type.

What should I do if expand_generated is true && isdefined(m, :generator) && mtypes isn't a leaf type? Should I just throw an error?

[JeffBezanson]

Yes, I think throwing an error is all you can do.

[jrevels]

This is complete (pending further review) and tests are passing (except for a Travis failure on OSX, which AFAICT is unrelated).

[jrevels]

I plan on merging this tomorrow afternoon barring any further change requests.