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

[Peter Smith via llvm-commits]

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@@ -0,0 +1,132 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -n -filetype=obj -triple=x86_64 %s -o %t.o
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smithp35 wrote:

I think it will be worth adding some tests for symbols/sections that are kept via linker-script KEEP, referenced from a linker script expression.

The referenced from a linker script expression is something that might surprise some users. It would be useful to see what that prints.

Although it could be an extension in a later patch, I think we could give more information about the top level symbol. For example
```
>>> kept live by _start (crt0.o(.text.start); entry point)
```
Could also mention exported symbol, retained, referenced from linker script etc.


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