Remove Linkage::Private

This is the same as Linkage::Internal except that it doesn't emit any
symbol. Some backends may not support it and it isn't all that useful
anyway.

compiler/rustc_codegen_llvm/src/base.rs

@@ -159,7 +159,6 @@ pub(crate) fn linkage_to_llvm(linkage: Linkage) -> llvm::Linkage {
         Linkage::WeakODR => llvm::Linkage::WeakODRLinkage,
         Linkage::Appending => llvm::Linkage::AppendingLinkage,
         Linkage::Internal => llvm::Linkage::InternalLinkage,
-        Linkage::Private => llvm::Linkage::PrivateLinkage,
         Linkage::ExternalWeak => llvm::Linkage::ExternalWeakLinkage,
         Linkage::Common => llvm::Linkage::CommonLinkage,
     }