Disabled demangling for now, to break the dependency: Exeception -> Units -> Algorithms -> Range -> Concepts. UGH!
Remove obsolete MSVC check from pragma guard

 git grep -h -B1 "^#\s*pragma once" | grep -v pragma | sort | uniq

is now clean.
Exception: Remove obsolete MSVC version check.
Exception: Remove obsolete GCC version checks.