[clang] [clang-tools-extra] [clang-tidy] Add new check `modernize-use-designated-initializers` (PR #80541)

[Danny Mösch via cfe-commits]

================

----------------
SimplyDanny wrote:

I want to reuse this function from Clangd (which provides inlay hints for aggregate types).

Not sure if this is the right place for such common functionality. Please guide me to find a better location if it doesn't fit well here.

For review, I haven't touch the code itself, but just moved it. Only the documentation of the public function `getDesignators` has been extended.

https://github.com/llvm/llvm-project/pull/80541