Auto merge of #36200 - mattico:fix-llvm-linkage, r=arielb1

Fix incorrect LLVM Linkage enum

Followup of #33994 to actually work.

The `Linkage` enum in librustc_llvm got out of sync with the version in LLVM and it caused two variants of the `#[linkage=""]` attribute to break.

This adds the functions `LLVMRustGetLinkage` and `LLVMRustSetLinkage` which convert between the Rust Linkage enum and the LLVM one, which should stop this from breaking every time LLVM changes it.

Possible remaining concerns:

1. There could be a codegen test to make sure that the attributes are applied correctly (I don't know how to do this).
2. ~~The test does not exercise the `appending` linkage. I can't figure out how to make a global static raw pointer to an array. This might not even be possible? If not we should probably remove appending linkage as its unusable in rust.~~ Appending linkage is not 'emittable' anyway.
3. The test only runs on Linux.

Fixes #33992

r? @alexcrichton

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