Source control for all dmd source

[Sean Kelly]

Jason House wrote:
> Over in D.anounce, the LDC devs said they would have an easier time upgrading to newer dmd (fe) versions if the source was in source control. Even if Walter is the only one with write access, it's still be helpful. It's helpful for more than just the LDC folks; that's just the most recent example.

It would be nice to have DMD in version control, but I don't buy the LDC 
argument.  It's trivial to diff one release against another, regardless 
of whether version control is involved.