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

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

@AaronBallman 
Tt seems `SetBuffered` is not honored for stderr intentionally :

Here we return 0
https://github.com/llvm/llvm-project/blob/e68a3e4d0dd349a34c02471438d2e97c2b29e846/llvm/lib/Support/raw_ostream.cpp#L858
To not set the buffer here
https://github.com/llvm/llvm-project/blob/e68a3e4d0dd349a34c02471438d2e97c2b29e846/llvm/lib/Support/raw_ostream.cpp#L105

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