[AST] Make PatternBindingInitializer::setBinding private

And add some assertions to ensure we don't attempt
to re-set the binding or index (the parent context
can unfortunately change currently).

Author: hamishknight

include/swift/AST/Initializer.h

@@ -76,6 +76,11 @@ class PatternBindingInitializer : public Initializer {
   // created lazily for 'self' lookup from lazy property initializer
   ParamDecl *SelfParam;
 
+  // Sets itself as the parent.
+  friend class PatternBindingDecl;
+
+  void setBinding(PatternBindingDecl *binding, unsigned bindingIndex);
+
   explicit PatternBindingInitializer(DeclContext *parent)
     : Initializer(InitializerKind::PatternBinding, parent),
       Binding(nullptr), SelfParam(nullptr) {
@@ -90,8 +95,6 @@ class PatternBindingInitializer : public Initializer {
   static PatternBindingInitializer *createDeserialized(PatternBindingDecl *PBD,
                                                        unsigned index);
 
-  void setBinding(PatternBindingDecl *binding, unsigned bindingIndex);
-
   PatternBindingDecl *getBinding() const { return Binding; }
 
   unsigned getBindingIndex() const { return SpareBits; }

lib/AST/Decl.cpp

@@ -2004,6 +2004,11 @@ ParamDecl *PatternBindingInitializer::getImplicitSelfDecl() const {
 
 void PatternBindingInitializer::setBinding(PatternBindingDecl *binding,
                                            unsigned bindingIndex) {
+  assert(binding);
+  assert(!Binding || Binding == binding &&
+         "Cannot change the binding after the fact");
+  assert(!Binding || SpareBits == bindingIndex &&
+         "Cannot change the binding index after the fact");
   setParent(binding->getDeclContext());
   Binding = binding;
   SpareBits = bindingIndex;