[Demangle] Add another test for ItaniumPartialDemangler

Summary: Show the behavior of print operations in the ItaniumPartialDemangler. It's a summary of what the current integration in LLDB assumes. For new users this may be a useful example.

Reviewers: erik.pilkington

Subscribers: llvm-commits, lldb-commits

Differential Revision: https://reviews.llvm.org/D50473

llvm-svn: 339297