[clang] [clang-repl] add %help, documentation, and tests for %commands (PR #150348)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> Please update the PR description to reflect the current changes.
> 
> @vgvassilev what do you think of this?

Quite nice piece of work. Than you both for doing this!

> 
> Only question I have is whether there's a special way to print non-error output, or whether stdout is fine? In theory `%help` is just like `printf("..."); but it prints some set text.

I am not aware of any other way to achieve that and maybe stdout is just fine.


https://github.com/llvm/llvm-project/pull/150348