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

[via llvm-commits]

Sirraide 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.

Hmm, considering that `TextDiagnostic` is the thing that both emits the colours and cares about the column count, I feel like it should be the one to create the formatted stream.

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