<rdar://problem/15201312>

Inlined a copy of cxa_demangle.cpp from:

http://llvm.org/svn/llvm-project/libcxxabi/trunk/src/cxa_demangle.cpp

For systems that don't have demangling built into the system, and for systems that don't want to use the version that is installed. Defining LLDB_USE_BUILTIN_DEMANGLER in your build system allows you to use the built in demangler. This setting is curently automatically enabled for Windows builds.