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

[Daniel Thornburgh 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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mysterymath wrote:

I added to the whyLive interface a place for string fields; accordingly, all liveness edges have recorded reasons, including the top level. The interface is such that it's difficult to forget to provide a reason.

With this change, linker script cases don't vary from the ones tested semantically; it's just that different reason strings are used. We could exhaustively test all the reason strings, but that doesn't seem very likely to catch additional issues given the nature of the interface.

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