[clang][Interp] DerivedToBase casts

[shafik]

LGTM

clang/lib/AST/Interp/ByteCodeExprGen.cpp

return dereference(
         [this, CE](PrimType T) {
           // Pointer on stack - dereference it.
           if (!this->emitLoadPop(T, CE))
             return false;
           return DiscardResult ? this->emitPop(T, CE) : true;
         });
   }
 
-  case CK_UncheckedDerivedToBase: {
+  case CK_UncheckedDerivedToBase:
+  case CK_DerivedToBase: {
     if (!this->visit(SubExpr))
       return false;
 
     const CXXRecordDecl *FromDecl = getRecordDecl(SubExpr);
     assert(FromDecl);
     const CXXRecordDecl *ToDecl = getRecordDecl(CE);
     assert(ToDecl);
     const Record *R = getRecord(FromDecl);

clang/test/AST/Interp/records.cpp

namespace MI {
   class C : public A, public B {
   public:
     constexpr C() : A(10), B(20) {}
   };
   constexpr C c = {};
   static_assert(c.a == 10, "");
   static_assert(c.b == 20, "");
 
+  constexpr const A *aPointer = &c;
+  constexpr const B *bPointer = &c;
 
   class D : private A, private B {
     public:
     constexpr D() : A(20), B(30) {}
     constexpr int getA() const { return a; }
     constexpr int getB() const { return b; }
   };
   constexpr D d = {};