Generate unique identifiers for Decl objects

[NoQ]

We're getting pretty good at it ^^

include/clang/AST/DeclBase.h

public:
   // Debuggers don't usually respect default arguments.
   void dump() const;
 
   // Same as dump(), but forces color printing.
   void dumpColor() const;
 
   void dump(raw_ostream &Out, bool Deserialize = false) const;
 
+  /// \return Unique reproducible object identifier
+  int64_t getID() const;
+
   /// Looks through the Decl's underlying type to extract a FunctionType
   /// when possible. Will return null if the type underlying the Decl does not
   /// have a FunctionType.
   const FunctionType *getFunctionType(bool BlocksToo = true) const;
 
 private:
   void setAttrsImpl(const AttrVec& Attrs, ASTContext &Ctx);
   void setDeclContextsImpl(DeclContext *SemaDC, DeclContext *LexicalDC,

lib/AST/DeclBase.cpp

assert(Access != AS_none &&
          "Access specifier is AS_none inside a record decl");
 #endif
   return true;
 }
 
 static Decl::Kind getKind(const Decl *D) { return D->getKind(); }
 static Decl::Kind getKind(const DeclContext *DC) { return DC->getDeclKind(); }
 
+int64_t Decl::getID() const {
+  Optional<int64_t> Out = getASTContext().getAllocator().identifyObject(this);
+  assert(Out && "Wrong allocator used");
+  assert(*Out % alignof(Decl) == 0 && "Wrong alignment information");
+  return *Out / alignof(Decl);
+}
+
 const FunctionType *Decl::getFunctionType(bool BlocksToo) const {
   QualType Ty;
   if (const auto *D = dyn_cast<ValueDecl>(this))
     Ty = D->getType();
   else if (const auto *D = dyn_cast<TypedefNameDecl>(this))
     Ty = D->getUnderlyingType();
   else
     return nullptr;