Ensure GetDemangledName's thread safety by making the demangle buffer thread specific.

I'm not sure when multiple threads enter this method but a race-condition
causing a crash in malloc can be reproduced with this little script:

echo file $(which lldb) > cmd
echo "run\nbreak set -n main\nrun\nexit" >> cmd
lldb -s cmd

It may need a few runs before it crashes though.

llvm-svn: 106544