[clang] Module global init mangling

C++20 modules require emission of an initializer function, which is
called by importers of the module.  This implements the mangling for
that function.  It is the one place the ABI exposes partition names in
symbols -- but fortunately only needed by other TUs of that same module.

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D122741