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

[Vassil Vassilev via cfe-commits]

https://github.com/vgvassilev approved this pull request.

LGTM! Probably more changes to the documentation is required but that's a good step! Thank you!

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