rustc: Remove __rust_crate_map_toplevel alias

[klutzy]

It was a hack to workaround llvm bug: when
_rust_crate_map_toplevel is dll-exported, llvm generated
-export:__rust_crate_map_toplevel,data where extra underscore
is due to unnecessary mangling. Then linker tries to find
__rust_crate_map_toplevel symbol, but it doesn't exist.
Therefore alias was added to prevent the link failure.
(See http://llvm.org/bugs/show_bug.cgi?id=10174 for details.)

It also caused LTO issue: llvm's LTO pass removes
__rust_crate_map_toplevel alias for some reason I don't know,
so build failed at link time.

Fixes #12471

[klutzy]

Relevant llvm patch: https://github.com/klutzy/llvm/tree/rust-fix-lto

[alexcrichton]

Does this require an patch to LLVM itself? If so, have you tried to upstream it first?

[klutzy]

Yes, llvm patch is needed. I haven't tried to upstream yet but I'll try it soon.

[alexcrichton]

I'm in the process of upstreaming the bulk of the rest of our patches, so I'd prefer to not merge this before seeing what upstream thinks about the LLVM patch.

That being said, thank you so much for finding this! I think LLVM will be quite receptive to your patch.

[adrientetar]

This submodule points towards rust-lang/llvm (it would not work with a commit hash from one of your repos).

[luqmana]

@klutzy yay! thanks for getting rid of that annoying wart

[alexcrichton]

Would it be possible to land the removal of the aliasing before the upgrade to LLVM?

[klutzy]

@alexcrichton Yep, it seems easier to use .def file which is standard way to dllexport symbols: prepare some .def file and give it to ld.

Anyway I found my llvm patch is wrong (e.g. it breaks msvc-compatibility behavior) so I'm closing this until new fix is done.