[llvm-cxxfilt] Split and demangle stdin input on certain non-alphanumerics.

Summary:
This patch attempts to replicate GNU c++-filt behavior when splitting stdin input for demangling.

Previously, cxx-filt would split input only on spaces.  Each delimited item is then demangled.
From what I have tested, GNU c++filt also splits input on any character that does not make
up the mangled name (notably commas, but also a large set of non-alphanumeric characters).

This patch splits stdin input on any character that does not belong to the Itanium mangling
format (since Itanium is currently the only supported format in llvm-cxxfilt).

This is an update to PR39990

Reviewers: jhenderson, tejohnson, compnerd

Reviewed By: compnerd

Subscribers: erik.pilkington, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58416

llvm-svn: 354998