Following the discussion on the emacs-devel mailing list, here is
the updated patch based on the emacs-29 branch.

- Update vhdl-compiler definition and docstring to include the
  message type.

- Increment vhdl-version to 3.38.5 because of the above
  change. Skip several versions due to divergent development.

- Set the type of all other vhdl-compiler-alist entries to nil to
  explicitly declare them as error messages.

- Update the Xilinx Vivado compiler entry's library directory, it
  must end with '/'.

By defining the error message type to be nil rather than 2, we
can forego the backwards compatibility code.