The dmd backend outta be a git sub module.

[Walter Bright]

On 7/1/2020 11:06 AM, 12345swordy wrote:
> Having it as a sub module allows walter to full speed ahead without waiting for 
> Pull request approval. I don't see other backend developers active besides 
> walter here, on github.

Fortunately, Nick Wilson has been very pro-active in approving my PRs for the 
AArch64 backend, without running any sort of AArch64 tests. Because there aren't 
any AArch64 tests yet.

The only requirement he imposes is it doesn't break anything.

This special case for approvals enables me to keep the AArch64 work completely 
in sync with the dmd master code.