[demangler] Preserve line numbering in copied demangler sources

While prepending lines to the copied source files is functional, it
disturbs the line numbering between the original and the copy.  That
makes development more awkward than necessary, as it is the copy that
generally gets compiled first and emits compiler errors.

This uses sed to alter the first two lines, and also emits better
emacs mode setting, getting both C++ mode and read-only mode.

While here, also update and clarify documentation.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D118135