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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

This feels like we're hacking around a deeper issue with raw_ostream; I would have expected that `SetBuffered()` is honored. But it also worries me that we're manually enabling colors in all circumstances and no tests broke.

I'm also a bit worried we're updating `TextDiagnostic` but not `SARIFDiagnostic`; does emitting to SARIF also have interleaving issues?

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