Source compatibility failures on swift-4.2-branch

I noticed that we had several failures in the source compatibility suite when using compilers built from swift-4.2-branch. It looks like most are cases where we just didn't update projects.json to XFAIL things that we already knew were failing with master.

However, there are two failures that are not already XFAILed on master, and I am wondering if anyone has any insight into them. Perhaps a couple fixes need to be cherry-picked to swift-4.2-branch?

CoreStore: unsupported relocation with subtraction expression, symbol '_$S9CoreStore5WhereVMn' can not be undefined in a subtraction expression

RxDataSources: <unknown>:0: note: symbol '_$S7RxSwift27RecursiveImmediateSchedulerCMn' can not be undefined in a subtraction expression

Hi Mark,

Sorry I didn't see this sooner. It's probably this bug. I think there was a PR on master that fixed it: