Fixing C's Biggest Mistake

[rikki cattermole]

I've had some time to think about some more technical debt that could be 
paid off relatively easily.

In my last attempt at fixing ModuleInfo exportation, Iain complained 
about dmd's glue code state infecting the AST.

I.e. 
https://github.com/dlang/dmd/blob/5dfc20e016850820036a81a3c40b01bf08b8c244/compiler/src/dmd/dsymbol.h#L177

Which could be extracted out into its own struct which could be turned 
opaque or even used by ldc/gdc. The main issue is dub, which I think 
might be solvable by having a gluedefinitions subPackage in turn 
versions on if you have dmd-be.