Add a new "lldb" log channel named "demangle". If we have crashes that are...

Add a new "lldb" log channel named "demangle". If we have crashes that are related to demangling, we now can enable this logging and we will be able to reproduce demangler crashes (usually due to overflowing the stack) without needing someone's project.

<rdar://problem/25221899>                                                                     

llvm-svn: 270941