Don't include json_loader.h in ir.h

[sethfowler]

Here's a PR I've summoned from an old branch.

p4c takes a long time to compile, and one of the biggest factors is duplicate template instantiations in different translation units. We've worked around this to some degree with unified builds, but it'd be much better to resolve the underlying problem.

This patch takes one small step in that direction by eliminating the need to include json_loader.h in ir.h. This is accomplished by adding forward declarations for JSONLoader where needed (including in ir-generated.cpp) and moving the code that references JSONLoader from ir-inline.h into json_loader.h itself.

I had tried to do the same for json_generator.h, but while I have a patch that does it, it requires some icky explicit template instantiations. Getting rid of it cleanly would require a redesign that didn't implement toJSON() as a virtual method.

[ChrisDodd]

Does this really make much speed difference? It only removes the templates from compilation units that never instantiate them, so the only savings is the time to read and parse one file...

[sethfowler]

No, it doesn't make much speed difference. It's just a baby step towards the larger goal of minimizing the amount of code that gets #include'd in each file.