[clang] [clang-format][NFC] Eliminate the IsCpp parameter in all functions (PR #84599)

[Owen Pan via cfe-commits]

owenca wrote:

> That's okay for clang-format, but you couldn't use libFormat with different languages at once anymore.

I believe we still could because `IsCpp` is set to `Style.isCpp()` every time new instances of the classes are constructed. I'll add a couple of test cases to demonstrate this.

> I dislike global state in libraries.

So do I, but I prefer this to keeping the `IsCpp` parameter. See also https://github.com/llvm/llvm-project/pull/83709#discussion_r1512657115.

https://github.com/llvm/llvm-project/pull/84599