Moving druntime into the DMD repository

[Steven Schveighoffer]

On 7/27/18 7:03 AM, Seb wrote:
> This a thread to explore whether it would be feasible to do so.
> 
> Motivation
> ----------
> 
> DRuntime and DMD heavily depend on each other.
> 
> It happens very often that a PR needs to touch both and then a 
> complicated three-step (or sometimes four-step PR series) needs to be 
> done to keep the CIs happy.
> Moreover, as the whole testsuite is at dmd, it happens more often that 
> an error is caught in the testsuite, but would require a druntime PR or 
> the opposite you make a PR to druntime and want its accompanying test to 
> be checked by the CI.

The auto tester is home-grown. We can adjust it if needed to test PRs in 
tandem.

In fact, I believe we used to use Daniel Murphy's branch to test mutual 
PRs this way (kind of ad-hoc, but it worked, and was not frequently needed).

I'd MUCH rather not merge these two repos. Note that this sometimes even 
happens with Phobos too!

If needed, I'd rather there be an interface on the auto-tester to tag 
which mutual PR is needed from any of the other repos.

-Steve