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

[Björn Schäpers via cfe-commits]

HazardyKnusperkeks wrote:

> > What happens if you use libFormat concurrently?
> 
> Can you elaborate? Doesn't each process get its own copy of the globals of a shared library?

I mean multiple threads in the same process with different languages. maybe unlikely, but not impossible.

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