[clang] Fix returning the underlying VarDecl as top-level decl for VarTemplateDecl.

Given the following VarTemplateDecl AST,

```
VarTemplateDecl col:26 X
|-TemplateTypeParmDecl typename depth 0 index 0
`-VarDecl X 'bool' cinit
  `-CXXBoolLiteralExpr 'bool' true
```

previously, we returned the VarDecl as the top-level decl, which was not
correct, the top-level decl should be VarTemplateDecl.

Differential Revision: https://reviews.llvm.org/D89098