[llvm-cxxfilt] Split and demangle stdin input

Summary:
Originally, llvm-cxxfilt would treat a line as a single mangled item to be demangled.
If a mangled name appears in the middle of that string, that name would not be demangled.

GNU c++filt  splits and demangles every word  in a  string that is piped to it via stdin.
Prior to this patch llvm-cxxfilt would never split strings  piped to it.
This patch replicates the GNU behavior and splits strings that are piped to it via stdin.

This fixes PR39990

Reviewers: compnerd, jhenderson, davide

Reviewed By: compnerd, jhenderson

Subscribers: erik.pilkington, jhenderson, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57350

llvm-svn: 353743