vNext: Make the abi-breaking-changes branch available on GitHub

There are a number of issues out there (both listed here on the STL repository and also things like https://developercommunity.visualstudio.com/content/problem/193041/the-visual-c-2017-condition-variable-implementatio.html listed on the Visual C++ bug tracker) that are listed as being fixable but only in whatever the next release is that breaks ABI.
There are likely people who want those fixes and would be quite happy using their own STL build that broke the ABI if it meant getting those fixes sooner.

I suggest you consider releasing the ABI breaking changes in a way that those who know what they are doing and want something that breaks ABI can get them. (but where anyone who doesn't explicitly download the ABI-breaking-changes branch wont have to worry) Or if its not possible to publish this stuff (either because there are also ABI changes in vcruntime, vcstartup, concrt or other non-open bits or because vcruntime, vcstartup, concrt or other non-open bits need recompiling with the new ABI-breaking code to pick up the new ABI) then at least consider publishing a comprenensive accurate list of the things that have/can be fixed but only by breaking ABI (so people know where the STL has bugs (or differences with the C++ standards and why they can't be fixed at this time)

Start by reviewing issue #169 and its discussion, including the linked Visual C++ bug-tracker report, to understand the ABI-breaking fixes being requested. Determine whether the repository can publish the ABI-breaking branch or needs an accurate list of affected fixes; completion requires an agreed approach and the corresponding public release information.