[lld] [lld][ELF] Add --why-live flag (inspired by Mach-O) (PR #127112)

[Fangrui Song via llvm-commits]

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@@ -751,6 +751,8 @@ Report unresolved symbols as warnings.
 Force load of all members in a static library.
 .It Fl -why-extract Ns = Ns Ar file
 Print to a file about why archive members are extracted.
+.It Fl --why-live Ns = Ns Ar glob
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MaskRay wrote:

Fl contains a `-` ...

https://github.com/llvm/llvm-project/pull/127112