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

[Owen Pan via cfe-commits]

https://github.com/owenca commented:

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

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