[clang][AST][ASTImporter] improve AST comparasion on VarDecl & GotoStmt (#66976)

improve AST comparasion on VarDecl & GotoStmt:
  1. VarDecl should not be ignored,
  2. GotoStmt has no children, it should be handle explicitly.

Reviewed By: donat.nagy

Differential Revision: https://reviews.llvm.org/D159519



Co-authored-by: huqizhi <836744285@qq.com>