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

[Owen Pan via cfe-commits]

owenca wrote:

> I don't understand this fix, it needs tests to show what you are doing

See https://github.com/llvm/llvm-project/pull/143249#discussion_r2136966951 and https://github.com/llvm/llvm-project/pull/143249#issuecomment-2957775336. IMO, the current `BraceWrapping.IndentBraces` boolean is easy to understand: Indent (or not) the wrapped braces. I don't know if it makes sense to change it to an `enum` or `struct`, move it out of `BraceWrapping`, or add an `IndentLambdaBraces` to accommodate a very specific style in #143248.

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