Improve Incremental Compile Time of Projects that are split up into several Frameworks

[fabb (Fabian)]

Looks like the „module stability“ project could help on this front, since as a side effect it could improve incremental build times:

Plan for module stability

As you can probably tell, I've been saving a lot of discussions, waiting for Swift 4.2 to be in a pretty good place before we jump all in on Swift 5. This one's one of the main things I plan to be working on in the next year or so, so I wanted to make sure you all had the big picture. It is another massive Jordan Rose post, so if you want the TLDR, just read the bolded sentences in each section and then jump down to the plan at the end. Thanks! Introduction ABI stability means that an executab…