[clang][ASTVisitor] Visit `HoldingVar` from `BindingDecl`. (#117858)
Tuple-like types introduce `VarDecl`s in the AST for their "holding
vars", but AST visitors do not visit those. As a result the `VarDecl`
for the holding var is orphaned when trying to retreive its parents.
Fix a `FlowSensitive` test that assumes that only a `BindingDecl` is
introduced with the given name (the matcher now can also reach the
`VarDecl` for the holding var).