Some precursor work to adding module demangling.
- some mismatched comment and code in the demangler
- a const fn was not marked thusly
- we use std::islower. A direct range check is smaller code (no function call), and we know we're in ASCII-land and later in that same function make the same assumption about upper-case contiguity. Heck, maybe just drop the switch's precondition and rely on the optimizer to do its thing?
- the directory is cloned in two places, which had gotten out of sync. AFAICT there is no need for the different header protection macros. Perhaps, now this is a monorepo we can elide the need for a copy. Symlinks would be simplest to achieve that, but I don't think they're available on all file systems being built on?