Various LTO cleanups

[bjorn3]

• Move some special casing of thin local LTO into a single location.
• Move lto_import_only_modules handling for fat LTO earlier. There is no reason to keep it separate until right before pass the LTO modules to the codegen backend. For thin LTO this introduces ThinLtoInput to correctly handle incr comp caching.
• Remove the Linker type from cg_llvm. It previously helped deduplicate code for -Zcombine-cgus, but that flag no longer exists.

Part of rust-lang/compiler-team#908

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

[rustbot]

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[antoyo]

Before we introduce more LTO changes, would it be possible to fix this issue introduced by your previous LTO change please?

It would be nice to also add a test for this specific scenario in the Rust CI.

[bjorn3]

I missed that comment. Will take a look.

[bjorn3]

@rustbot block until cg_gcc is unblocked

[bjorn3]

@rustbot blocked