[AST] Generate unique identifiers for CXXCtorInitializer objects.

[NoQ]

Summary

This follows D51822 and D52113 to add a cheap way of obtaining a unique and relatively stable* numeric identifier for CXXCtorInitializers.

__
*I.e., reproducible across runs but not necessarily across different host machines.

[george.karpenkov]

lib/AST/DeclCXX.cpp

2249

Should we factor out this code? It's a fourth duplication now, probably a generic form should be in the allocator.

[NoQ]

Rebase on top of D54486!

[NoQ]

Wait, i already did that in D54488.

include/clang/AST/DeclCXX.h

CXXCtorInitializer(ASTContext &Context, IndirectFieldDecl *Member,
                      SourceLocation MemberLoc, SourceLocation L, Expr *Init,
                      SourceLocation R);
 
   /// Creates a new delegating initializer.
   explicit
   CXXCtorInitializer(ASTContext &Context, TypeSourceInfo *TInfo,
                      SourceLocation L, Expr *Init, SourceLocation R);
 
+  /// \return Unique reproducible object identifier.
+  int64_t getID(const ASTContext &Context) const;
+
   /// Determine whether this initializer is initializing a base class.
   bool isBaseInitializer() const {
     return Initializee.is<TypeSourceInfo*>() && !IsDelegating;
   }
 
   /// Determine whether this initializer is initializing a non-static
   /// data member.
   bool isMemberInitializer() const { return Initializee.is<FieldDecl*>(); }

lib/AST/DeclCXX.cpp

 
 CXXCtorInitializer::CXXCtorInitializer(ASTContext &Context,
                                        TypeSourceInfo *TInfo,
                                        SourceLocation L, Expr *Init,
                                        SourceLocation R)
     : Initializee(TInfo), Init(Init), LParenLoc(L), RParenLoc(R),
       IsDelegating(true), IsVirtual(false), IsWritten(false), SourceOrder(0) {}
 
+int64_t CXXCtorInitializer::getID(const ASTContext &Context) const {
+  Optional<int64_t> Out = Context.getAllocator().identifyObject(this);
+  assert(Out && "Wrong allocator used");
+  assert(*Out % alignof(CXXCtorInitializer) == 0 &&
+         "Wrong alignment information");
+  return *Out / alignof(CXXCtorInitializer);
+}
+
 TypeLoc CXXCtorInitializer::getBaseClassLoc() const {
   if (isBaseInitializer())
     return Initializee.get<TypeSourceInfo*>()->getTypeLoc();
   else
     return {};
 }
 
 const Type *CXXCtorInitializer::getBaseClass() const {