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

[Owen Pan via cfe-commits]

owenca wrote:

@mydeveloperday requested it. See https://github.com/llvm/llvm-project/pull/91317#pullrequestreview-2095010527.

There are calls to `getStyle()` in `ConfigParseTest.cpp`. Maybe add a test there?

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