Tests crash from inappropriate use of common linkage

Normally, variables with common linkage must be zero-initialized. In Rust,
common linkage variables that are not zero-initialized causes a crash in the
compiler backend.

This commit adds a test case to confirm this behavior, which will inform us if
it changes in the future.

tests/ui/linkage-attr/common-linkage-non-zero-init.rs

@@ -0,0 +1,14 @@
+// build-fail
+// failure-status: 101
+// known-bug: #109681
+
+// This test verifies that we continue to hit the LLVM error for common linkage with non-zero
+// initializers, since it generates invalid LLVM IR.
+// Linkages are internal features marked as perma-unstable, so we don't need to fix the issue
+// for now.
+#![crate_type="lib"]
+#![feature(linkage)]
+
+#[linkage = "common"]
+#[no_mangle]
+pub static TEST: bool = true;

tests/ui/linkage-attr/common-linkage-non-zero-init.stderr

@@ -0,0 +1,3 @@
+'common' global must have a zero initializer!
+ptr @TEST
+LLVM ERROR: Broken module found, compilation aborted!