Remove clang-pseudo (#109154)
The functionality is incomplete and the authors have since shifted gears
to other work, so this is effectively unmaintained.
The original design document for clang-pseudo can be found at:
https://docs.google.com/document/d/1eGkTOsFja63wsv8v0vd5JdoTonj-NlN3ujGF0T7xDbM/edit
in case anyone wishes to pick this project back up again in the future.
Original RFC: https://discourse.llvm.org/t/removing-pseudo-parser/71131/
