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

[via cfe-commits]

rmarker wrote:

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

This doesn't fix anything. Rather, it is upgrading `IndentBraces` to be a struct so that more options to control the indentation can be added to it in a subsequent PR. For example #143249 as a possibility.

The tests didn't end up being touched as the `IndentBraces` option currently isn't used in the tests and therefore didn't need to be updated. I imagine this is because, at the moment, its functionality is covered by the `GNU` brace wrapping tests, which use `IndentBraces` under the hood. New tests for the option would need to be added when extra options for indenting are added.
Though, if you think it is worthwhile, I could add initial tests to this PR based on the `GNU` brace wrapping tests? They would then act as a base for any subsequent PRs to build upon.

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