The purpose of this patch is not so much a review but to provide an NFC version of the patch that was submitted to create r342218. See the initial review in https://reviews.llvm.org/D51081.
The original patch caused TSan failures and this is an attempt to narrow down the cause.
Basically, the difference from the original patch is that it does not change any of the current format for dumping rangelists.
It is also rebased vs. Dave's latest patch supporting RLE_startx etc.