[llvm] Revert "[MC] Explicitly mark MCSymbol for MO_ExternalSymbol" (PR #133291)

[weiwei chen via llvm-commits]

weiweichen wrote:

> I'm not sure what the actual goal of the original patch is. If a symbol is not defined, it defaults to being external. If a symbol is defined, we emit the directives for the definition at the point of the definition. So I'm not sure what effect you're hoping for, besides breaking Julia's hack.

The goal was definitely not to break Julia's hack 🙊. We (Mojo) were trying to do a MC level linking so that we break llvm module into multiple submodules to compile and codegen in parallel (technically into *.o files with symbol linkage type change), but instead of making them into one `.a` file, we want to fix the symbol linkage type and still produce one `*.o` file. What we did is to run the codgen pipeline to generate MC first in parallel, and then AsmPrint in one piece. This change is needed for us to do so to make ORC JIT happy (just producing a `*.o` file for executable is also fine without this). 

Granted, it was a bit hacky on our side too (I'm definitely not a MC expert and was hacking my way through our un-conventional use of the backend here). There is probably also some nuance on ORC JIT side why without this fix, it doesn't work. I was mostly looking at `MO_ExternalSymbol` and think it should be straightforward to be `extern` (why would it not be?) and put in the commit (and yes, I should've added a test).

Hope this provides some context on the change. I'm going to give a talk about the details on what we did next month at EuroLLVM. Would definitely love to hears suggestions/advices on how to make this change better and more disciplined, and I'm also in the process of preparing upstream PRs to show how our parallel strategy looks like).

Both PRs are WIP:
https://github.com/llvm/llvm-project/pull/121543
https://github.com/llvm/llvm-project/pull/132989

An early talk about the general idea: https://youtu.be/yuSBEXkjfEA?si=HjgjkxJ9hLfnSvBj&t=813

https://github.com/llvm/llvm-project/pull/133291