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

[Fangrui Song via llvm-commits]

https://github.com/MaskRay commented:

If the performance issue is severe, we might need to use CRTP to abstract away the graph construction part and incur the overhead when the feature is used.

https://reviews.llvm.org/D69607 is a previous attempt adding the option. 

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