rustc_symbol_mangling: Address all rustc::potential_query_instability lints

Instead of allowing `rustc::potential_query_instability` on the whole
crate we go over each lint and allow it individually if it is safe to
do. Turns out there were no instances of this lint in this crate.

Author: Enselic

compiler/rustc_symbol_mangling/src/lib.rs

@@ -93,7 +93,6 @@
 #![allow(internal_features)]
 #![feature(never_type)]
 #![recursion_limit = "256"]
-#![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]