Glow: Minimize #ifdef FACEBOOK_INTERNAL in GlowJIT

[bertmaher]

PR #1357 introduced some #ifdefs to GlowJIT.{h,cpp} to work around the shifting ORC JIT API in llvm 7.0. We can probably minimize those by:

• switching to the C API, if that is stable (OrcBindings.h, as mentioned by @qcolombet in #1357)
• thinning out the GlowJIT API (with a bit of work we can actually remove some of its API functions and types, so there's less to guard)
• figuring out the llvm 7.0 API for GDB registration listeners so we can do something sensible with NotifyLoadedFunctor

And at some point the LLVM 7.0 API will stabilize and we can shift to major version guards instead of FACEBOOK_INTERNAL.

[vedanuj]

@bertmaher Can I take a stab at this if no one else is working on it?

[bertmaher]

@vedanuj Be my guest!