[demangler] write-protect non-canonical source

To try and avoid undesired changes to the non-canonical demangler
sources, change the cp-to-llvm script to (a) write-protect the target
files and (b) prepend 'do not edit' comments that are significant to
emacs[*], and hopefully humans.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D118008