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

[via llvm-commits]

https://github.com/alx32 created https://github.com/llvm/llvm-project/pull/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. 

>From ba3958d92eae9f7931703c283cfc58b55e51f466 Mon Sep 17 00:00:00 2001
From: Alex Borcan <alexborcan at fb.com>
Date: Fri, 17 Jan 2025 09:04:28 -0800
Subject: [PATCH] [lld-macho] Doccument '-icf' flag options

---
 lld/MachO/Options.td | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/lld/MachO/Options.td b/lld/MachO/Options.td
index 4b1e9e4391070d..39191af7dc1692 100644
--- a/lld/MachO/Options.td
+++ b/lld/MachO/Options.td
@@ -82,8 +82,12 @@ def print_dylib_search: Flag<["--"], "print-dylib-search">,
     HelpText<"Print which paths lld searched when trying to find dylibs">,
     Group<grp_lld>;
 def icf_eq: Joined<["--"], "icf=">,
-    HelpText<"Set level for identical code folding (default: none)">,
-    MetaVarName<"[none,safe,all]">,
+    HelpText<"Set level for identical code folding (default: none). Possible values:\n"
+            "  none        - Disable ICF\n"
+            "  safe        - Only folds non-address significant functions (as described by `__addrsig` section)\n"
+            "  safe_thunks - Like safe, but replaces address-significant functions with thunks\n"
+            "  all         - Fold all identical functions">,
+    MetaVarName<"[none,safe,safe_thunks,all]">,
     Group<grp_lld>;
 def keep_icf_stabs: Joined<["--"], "keep-icf-stabs">,
     HelpText<"Generate STABS entries for symbols folded by ICF. These entries can then be used by dsymutil to discover the address range where folded symbols are located.">,