[llvm-cxxfilt] Re-enable split and demangle stdin input on certain non-alphanumerics.
This restores the patch that splits demangled stdin input on
non-alphanumerics. I had reverted this patch earlier because it broke
Windows build-bots. I have updated the test so that it passes on
I was running the test from powershell and never saw the issue until I
switched to the mingw shell.
This reverts commit 628ab5c6820bdf3bb5a8e494b0fd9e7312ce7150.