[DDMD] Add backend-independent stub glue layer

This draws a line showing where the common frontend ends and the glue layer begins.  It can serve as a starting point for adding a new backend (if anybody is ever [so crazy](http://forum.dlang.org/thread/yfmgvgprfpiquakiyjlk@forum.dlang.org)) and allows for easy removal of the digital mars backend and its redistribution problems, if anybody wanted to use the frontend code for another tool.

Another use is checking fail-compilation cases properly fail during semantic analysis.

The stub can be activated by uncommenting the `#GLUEOBJ=gluestub.obj` line in win32.mak.

Currently win32 only.