[llvm] [DTLTO] Add remote compiler option forwarding coverage (PR #208589)

[Edd Dawson via llvm-commits]

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playstation-edd wrote:

May I suggest some renamings, which I think will help with readability?

`test.o` -> `elf.o`  (once you realize there's symmetry with `coff.o`, there are fewer hitches when reading).
`%{command}` -> `%{lto2-elf}`
`%{command-coff}` -> `%{lto2-coff}`

https://github.com/llvm/llvm-project/pull/208589