Implement Itanium name mangling support for C++ Modules TS.

This follows the scheme agreed with Nathan Sidwell, which can be found here:

  https://gcc.gnu.org/wiki/cxx-modules?action=AttachFile

This will be proposed to the itanium-cxx-abi list once we have some experience
with how well it works; the ABI for this TS should be considered unstable until
it is part of the Itanium C++ ABI.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312467 91177308-0d34-0410-b5e6-96231b3b80d8