[clang] [llvm] [clang] Use a formatted_raw_ostream in TextDiagnostic (PR #164935)

[Timm Baeder via llvm-commits]

tbaederr wrote:

Not sure if the `TextDiagnosticPrinter` should create the formatted stream from the `raw_ostream` or the `TextDiagnostic`. There is a unittest that passes `llvm::errs()` to the `TextDiagnostic` ctor directly.

https://github.com/llvm/llvm-project/pull/164935