[clang] [clang] Apply internal buffering to clang diagnostics printing (PR #113440)

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> Are we calling `SetUnbuffered` somewhere we shouldn't?

Well, yeah, but I think we should? 

https://github.com/llvm/llvm-project/blob/37832d5de2abb425c460375a4054478ce9f948fe/llvm/lib/Support/raw_ostream.cpp#L910

The thing is though, I've tried setting `SetBuffered` somewhere in `TextDiagnosticPrinter` and it didn't work out. Seems calling `SetBuffered` on `raw_ostream` doesn't do much.

https://github.com/llvm/llvm-project/pull/113440