[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
Windows.

I was running the test from powershell and never saw the issue until I
switched to the mingw shell.

This reverts commit 628ab5c6.

llvm-svn: 355031