[clang-tidy] Fix `readability-redundant-declaration` false positive for template friend declaration

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=48086 | PR#48086 ]]. The problem is that the current matcher uses `hasParent()` to detect friend declarations, but for a template friend declaration, the immediate parent of the `FunctionDecl` is a `FunctionTemplateDecl`, not the `FriendDecl`. Therefore, I have replaced the matcher with `hasAncestor()`.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D114299