The roll hasn't happened in a while so I had to bisect the CLs.
I tracked it down to https://reviews.llvm.org/D96435, specifically the increase in the size class map of the default num cached from 8 to 10 (eg: the CL works with 8, but not with 10).
This boggles me a bit because I thought I tested that back then, and also I don't see immediately why the increase here ends up triggering an access violation on Fuchsia.
Still trying to debug it.
As far as I can tell this works.
With a few extra fixes:
[==========] 183 tests from 87 test cases ran (10944 ms total). [ PASSED ] 183 tests
This helped unearthed a few things that I have to address in a separate CL.