[clang] [clang] Make verbose output streams configurable for embedded clients (PR #226022)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> I also considered the process-wide nature of this design. It would **not allow concurrent compiler sessions** within the same process or WebAssembly instance to select different informational streams. However, our xeus-cpp/JupyterLite and WasmBolt execution models serialize compiler invocations, and native kernels and Web Workers have isolated process or WebAssembly state. CppInterOp also currently uses a process-wide active-interpreter model rather than concurrent interpreters with independent output routing.

Not sure I follow. Any stream will have concurrency issues and in that sense `llvm::errs` and `llvm::outs` are no different.  The xeus-cpp and CppInterOp example can use the usual solution we have for `llvm::errs` -- how is that different?

https://github.com/llvm/llvm-project/pull/226022