Remove Linkage::Appending

It can only be used for certain LLVM internal variables like
llvm.global_ctors which users are not allowed to define.

compiler/rustc_codegen_llvm/src/base.rs

@@ -157,7 +157,6 @@ pub(crate) fn linkage_to_llvm(linkage: Linkage) -> llvm::Linkage {
         Linkage::LinkOnceODR => llvm::Linkage::LinkOnceODRLinkage,
         Linkage::WeakAny => llvm::Linkage::WeakAnyLinkage,
         Linkage::WeakODR => llvm::Linkage::WeakODRLinkage,
-        Linkage::Appending => llvm::Linkage::AppendingLinkage,
         Linkage::Internal => llvm::Linkage::InternalLinkage,
         Linkage::ExternalWeak => llvm::Linkage::ExternalWeakLinkage,
         Linkage::Common => llvm::Linkage::CommonLinkage,