[clang][ABI] New C++20 module mangling scheme

The existing module symbol mangling scheme turns out to be
undemangleable.  It is also desirable to switch to the
strong-ownership model as the hoped-for C++17 compatibility turns out
to be fragile, and we also now have a better way of controlling that.

The issue is captured on the ABI list at:
  https://github.com/itanium-cxx-abi/cxx-abi/issues/134

A document describing the issues and new mangling is at:
  https://drive.google.com/file/d/1qQjqptzOFT_lfXH8L6-iD9nCRi34wjft/view

This patch is the code-generation part.  I have a demangler too, but
that patch is based on some to-be-landed refactoring of the demangler.

The old mangling is unceremoniously dropped.  No backwards
compatibility, no deprectated old-mangling flag.  It was always
labelled experimental.  (Old and new manglings cannot be confused.)

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D118352