[clang] [clang-format] Add DiagHandler for getStyle function (PR #91317)

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pointhex wrote:

> Can you call `parseConfiguration()` instead of `getStyle()` in QtCreator while the user is entering code? It seems inefficient to call `getStyle()` all the time.

I do it in the places where I can use parseConfiguration instead. But in some cases I need getStyle and it's also called quite often. I have one optimisation in my mind, but I would like to have that patch as well anyway.

Am I right that no one has a major issue with the patch? So fixing the "const" issue and adding a unit test will let that in? 

https://github.com/llvm/llvm-project/pull/91317