[llvm-cxxfilt] Split and demangle stdin input
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
Differential Revision: https://reviews.llvm.org/D57350