[flang][OpenMP][NFC] Refactor to avoid global variable

I was really hoping this would also work for `hostEvalInfo` but
unfortunately that needed to be shared to a greater degree.

The same technique should work for that but it would need that class to
be made public and then the state kept between calls to
`genOpenMP*Construct`, which felt like more trouble than it was worth.

I'm open to abandoning this patch if solving one global variable doesn't
feel worth this much churn.

Making these changes I was wondering if we should implement this file
with one big class to wrap up all the state passed to every function.
Any thoughts?

Author: tblah