[Macros] When a macro defines a getter or setter, remove didSet/willSet

As with the initial value of a property that is converted from a stored
property to a computed property by an accessor macro, remove
didSet/willSet. It is the macro's responsibility to incorporate their
code or diagnose them.

Fixes rdar://111101833.

include/swift/AST/Decl.h

@@ -5238,6 +5238,8 @@ private:
 
     void addOpaqueAccessor(AccessorDecl *accessor);
 
+    void removeAccessor(AccessorDecl *accessor);
+
   private:
     MutableArrayRef<AccessorDecl *> getAccessorsBuffer() {
       return { getTrailingObjects<AccessorDecl*>(), NumAccessors };
@@ -5401,6 +5403,11 @@ public:
     return {};
   }
 
+  void removeAccessor(AccessorDecl *accessor) {
+    if (auto *info = Accessors.getPointer())
+      return info->removeAccessor(accessor);
+  }
+
   /// This is the primary mechanism by which we can easily determine whether
   /// this storage decl has any effects.
   ///
@@ -8906,6 +8913,9 @@ const ParamDecl *getParameterAt(const ValueDecl *source, unsigned index);
 /// nullptr if the source does not have a parameter list.
 const ParamDecl *getParameterAt(const DeclContext *source, unsigned index);
 
+StringRef getAccessorNameForDiagnostic(AccessorDecl *accessor, bool article);
+StringRef getAccessorNameForDiagnostic(AccessorKind accessorKind, bool article);
+
 void simple_display(llvm::raw_ostream &out,
                     OptionSet<NominalTypeDecl::LookupDirectFlags> options);