LDC to GDC, DMD merge

[IGotD-]

How does it work when doing changes to anything in LDC (compiler, 
library), the change is generic and you want it to be merged. 
Will this be merged to the repos of GDC and DMD, or if you want 
to do a generic change, do you have to use the DMD repo?

Another question, do we need LDC to have own repos for the 
compiler and library. Would it possible to use the DMD submodules 
and then have the LDC extras in an specific LDC submodule?