[lld-macho][NFC] Preserve original symbol isec, unwindEntry and size (#88357)

Currently, when moving symbols from one `InputSection` to another (like
in ICF) we directly update the symbol's `isec`, `unwindEntry` and
`size`. By doing this we lose the original information. This information
will be needed in a future change. Since when moving symbols we always
set the symbol's `wasCoalesced` and `isec-> replacement`, we can just
use this info to conditionally get the information we need at access
time.