RequirementMachine: Add reverse iterators over Term and MutableTerm

I thought I needed these for something but turns out that I don't,
but there's no harm in adding them since surely they will come up
eventually.

Author: slavapestov

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -687,14 +687,22 @@ struct Term::Storage final
 
 size_t Term::size() const { return Ptr->Size; }
 
-ArrayRef<Atom>::const_iterator Term::begin() const {
+ArrayRef<Atom>::iterator Term::begin() const {
   return Ptr->getElements().begin();
 }
 
-ArrayRef<Atom>::const_iterator Term::end() const {
+ArrayRef<Atom>::iterator Term::end() const {
   return Ptr->getElements().end();
 }
 
+ArrayRef<Atom>::reverse_iterator Term::rbegin() const {
+  return Ptr->getElements().rbegin();
+}
+
+ArrayRef<Atom>::reverse_iterator Term::rend() const {
+  return Ptr->getElements().rend();
+}
+
 Atom Term::back() const {
   return Ptr->getElements().back();
 }

lib/AST/RequirementMachine/RewriteSystem.h

@@ -256,9 +256,11 @@ class Term final {
 public:
   size_t size() const;
 
-  ArrayRef<Atom>::const_iterator begin() const;
+  ArrayRef<Atom>::iterator begin() const;
+  ArrayRef<Atom>::iterator end() const;
 
-  ArrayRef<Atom>::const_iterator end() const;
+  ArrayRef<Atom>::reverse_iterator rbegin() const;
+  ArrayRef<Atom>::reverse_iterator rend() const;
 
   Atom back() const;
 
@@ -344,6 +346,12 @@ class MutableTerm final {
   decltype(Atoms)::iterator begin() { return Atoms.begin(); }
   decltype(Atoms)::iterator end() { return Atoms.end(); }
 
+  decltype(Atoms)::const_reverse_iterator rbegin() const { return Atoms.rbegin(); }
+  decltype(Atoms)::const_reverse_iterator rend() const { return Atoms.rend(); }
+
+  decltype(Atoms)::reverse_iterator rbegin() { return Atoms.rbegin(); }
+  decltype(Atoms)::reverse_iterator rend() { return Atoms.rend(); }
+
   Atom back() const {
     return Atoms.back();
   }