[llvm] Add 'unifiedlto' option to gold plugin (PR #121336)

[Teresa Johnson via llvm-commits]

teresajohnson wrote:

@ormris can you take a look? In particular, I'm confused at how this gets enabled via lld. It appears there isn't a separate unified LTO option there, and the mode is set unconditionally to UnifiedThin or UnifiedRegular based on the `-flto=` option: https://github.com/llvm/llvm-project/blob/c7d237085bf9102ecf0c9105d8cc7fd94b752a3a/lld/ELF/Driver.cpp#L1651-L1662.

But then it looks like in the LTO code we will give an error if modules weren't built with -funified-lto: https://github.com/llvm/llvm-project/blob/c7d237085bf9102ecf0c9105d8cc7fd94b752a3a/llvm/lib/LTO/LTO.cpp#L768-L773

I can't recall how this is this supposed to work?

https://github.com/llvm/llvm-project/pull/121336