Introduce dedicated -Zdylib-lto flag for enabling LTO on dylibs

compiler/rustc_codegen_ssa/src/errors.rs

@@ -127,6 +127,9 @@ pub enum LinkRlibError {
 
     #[diag(codegen_ssa_rlib_not_found)]
     NotFound { crate_name: Symbol },
+
+    #[diag(codegen_ssa_rlib_incompatible_dependency_formats)]
+    IncompatibleDependencyFormats { ty1: String, ty2: String, list1: String, list2: String },
 }
 
 pub struct ThorinErrorWrapper(pub thorin::Error);