[Clang][AST] Fixed BindingDecl AST-dump for tuple like structures

The AST of a BindingDecl in case of tuple like structures wasn't
properly printed. For these bidnings there is information stored
in BindingDecl::getHoldingVar(), and this information was't
printed in the AST-dump.

Differential Revision: https://reviews.llvm.org/D126131