[lld-macho] Document '-icf' flag options (#123372)

Adding the `safe_thunks` option in `Options.td` as it was missing there
- mentioned by @Colibrow in
https://github.com/llvm/llvm-project/pull/106573
Also documenting what the various options mean. 

Help now looks like this:
```
..........
  --error-limit=<value>   Maximum number of errors to print before exiting (default: 20)
  --help-hidden           Display help for hidden options
  --icf=[none,safe,safe_thunks,all]
                          Set level for identical code folding (default: none). Possible values:
                            none        - Disable ICF
                            safe        - Only folds non-address significant functions (as described by `__addrsig` section)
                            safe_thunks - Like safe, but replaces address-significant functions with thunks
                            all         - Fold all identical functions
  --ignore-auto-link-option=<value>
                          Ignore a single auto-linked library or framework. Useful to ignore invalid options that ld64 ignores
  --irpgo-profile-sort=<profile>
                          Deprecated. Please use --irpgo-profile and --bp-startup-sort=function
..........
```

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