Support generating unique identifiers for Stmt objects

[george.karpenkov]

Summary

Unlike pointer values, those identifiers are reproducible between the runs.

[NoQ]

Looks as expected. I hope that assertion does actually hold. Should AST folks have a look as well?

clang/lib/AST/Stmt.cpp

310

Redundant empty line.

clang/include/clang/AST/Stmt.h

public:
 
   /// Dumps the specified AST fragment and all subtrees to
   /// \c llvm::errs().
   void dump() const;
   void dump(SourceManager &SM) const;
   void dump(raw_ostream &OS, SourceManager &SM) const;
   void dump(raw_ostream &OS) const;
 
+  /// \return Unique reproducible object identifier
+  int64_t getID(const ASTContext &Context) const;
+
   /// dumpColor - same as dump(), but forces color highlighting.
   void dumpColor() const;
 
   /// dumpPretty/printPretty - These two methods do a "pretty print" of the AST
   /// back to its original source language syntax.
   void dumpPretty(const ASTContext &Context) const;
   void printPretty(raw_ostream &OS, PrinterHelper *Helper,
                    const PrintingPolicy &Policy, unsigned Indentation = 0,

clang/lib/AST/Stmt.cpp

 #define STMT(type, base)                                                       \
   case Stmt::type##Class:                                                      \
     return static_cast<const type *>(this)->getEndLoc();
 #include "clang/AST/StmtNodes.inc"
   }
   llvm_unreachable("unknown statement kind");
 }
 
+int64_t Stmt::getID(const ASTContext &Context) const {
+  Optional<int64_t> Out = Context.getAllocator().identifyObject(this);
+  assert(Out && "Wrong allocator used");
+  assert(*Out % alignof(Stmt) == 0 && "Wrong alignment information");
+  return *Out / alignof(Stmt);
+
+}
+
 CompoundStmt::CompoundStmt(ArrayRef<Stmt *> Stmts, SourceLocation LB,
                            SourceLocation RB)
     : Stmt(CompoundStmtClass), LBraceLoc(LB), RBraceLoc(RB) {
   CompoundStmtBits.NumStmts = Stmts.size();
   setStmts(Stmts);
 }
 
 void CompoundStmt::setStmts(ArrayRef<Stmt *> Stmts) {