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

[Peter Smith via llvm-commits]

smithp35 wrote:

Apologies for the delay in responding. I've just started looking through the code. 

I think it would be helpful to update the description with a mini specification of what you intend the output to be. There will have been some design decisions made on what to include in the output. At the moment this needs to be reverse engineered out of the code and tests.

For example:
* Do we report all possible chains of reason for each symbol for each reloc or just the first one we find? 
* If we include all possible chains do we attempt to filter out chains that have the same parent section (.text.1 has multiple relocations to .text.2)?



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