[demangler][NFC] Small cleanups and sync

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.

Differential Revision: https://reviews.llvm.org/D117800

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