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_ssa/src/codegen_attrs.rs

@@ -49,7 +49,6 @@ fn linkage_by_name(tcx: TyCtxt<'_>, def_id: LocalDefId, name: &str) -> Linkage {
         "internal" => Internal,
         "linkonce" => LinkOnceAny,
         "linkonce_odr" => LinkOnceODR,
-        "private" => Private,
         "weak" => WeakAny,
         "weak_odr" => WeakODR,
         _ => tcx.dcx().span_fatal(tcx.def_span(def_id), "invalid linkage specified"),

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

@@ -187,7 +187,7 @@ fn prefix_and_suffix<'tcx>(
                     }
                 }
             }
-            Linkage::Internal | Linkage::Private => {
+            Linkage::Internal => {
                 // write nothing
             }
             Linkage::Appending => emit_fatal("Only global variables can have appending linkage!"),