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

src/test/run-pass/issue-33992.rs

@@ -0,0 +1,40 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-windows
+// ignore-macos
+
+#![feature(linkage)]
+
+#[linkage = "common"]
+pub static mut TEST1: u32 = 0u32;
+
+#[linkage = "external"]
+pub static TEST2: bool = true;
+
+#[linkage = "internal"]
+pub static TEST3: bool = true;
+
+#[linkage = "linkonce"]
+pub static TEST4: bool = true;
+
+#[linkage = "linkonce_odr"]
+pub static TEST5: bool = true;
+
+#[linkage = "private"]
+pub static TEST6: bool = true;
+
+#[linkage = "weak"]
+pub static TEST7: bool = true;
+
+#[linkage = "weak_odr"]
+pub static TEST8: bool = true;
+
+fn main() {}