Summary: [Folly] Cut config-time detection of `cplus_demangle_v3_callback` and the forward declarations and copy-pasting symbols, replacing them with `__has_include(<demangle.h>)`.

Reviewed By: Orvid

Differential Revision: D9287760

fbshipit-source-id: 4464ec22dd6f96cf081cda6051bebe4d41813390