Replace some instances of FxHashMap/FxHashSet with stable alternatives (mostly in rustc_hir and rustc_ast_lowering)

Part of https://github.com/rust-lang/compiler-team/issues/533

compiler/rustc_hir/src/pat_util.rs

@@ -1,7 +1,6 @@
 use crate::def::{CtorOf, DefKind, Res};
-use crate::def_id::DefId;
+use crate::def_id::{DefId, DefIdSet};
 use crate::hir::{self, BindingAnnotation, ByRef, HirId, PatKind};
-use rustc_data_structures::fx::FxHashSet;
 use rustc_span::symbol::Ident;
 use rustc_span::Span;
 
@@ -114,9 +113,9 @@ impl hir::Pat<'_> {
             }
             _ => true,
         });
-        // We remove duplicates by inserting into a `FxHashSet` to avoid re-ordering
+        // We remove duplicates by inserting into a hash set to avoid re-ordering
         // the bounds
-        let mut duplicates = FxHashSet::default();
+        let mut duplicates = DefIdSet::default();
         variants.retain(|def_id| duplicates.insert(*def_id));
         variants
     }