[clang] [clang-format][NFC] Upgrade IndentBraces option to a struct (PR #143663)

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rmarker wrote:

Probably not a big deal, the one ambiguity with the current `IndentBraces` is that it only indents the wrapped braces associated with `GNU`. It doesn't indent all braces that have the option of being wrapped, such as namespace, class or functions.

https://github.com/llvm/llvm-project/pull/143663