[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