Eliding many intra-comittee process details, the C++ working group
responsible for language evolution voted a paper as ready to forward to the
group responsible for standardese a paper adopting UAX 31 rules for the
syntax for identifiers, using XID_Start + LOW LINE and XID_Continue.

This replaces the white list of allowed code points from last millennium,
and fixes some mistakes like allowing RTL modifiers in identifiers.

Current draft of the paper is available at
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1949r6.html

I wanted to thank the Unicode committee for UAX 31 so that we didn't have
to reinvent the wheel.