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

[Aaron Ballman via cfe-commits]

AaronBallman 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

Good find! This was done in 177102271d3e7e453725cdcf9b28cdd288fe6c82, and I think our needs have changed in the intervening 15 years so that line buffering is perhaps now worth the added complexity. Maybe we should try backing this out and fixing the underlying issue? CC @sunfishcode for opinions

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