Improved the error message when we can find a function in the current program by printing the demangled name.

Also added the ability to create lldb_private::Mangled instances with a ConstString when we already have a ConstString for a mangled or demangled name. Also added the ability to call SetValue with a ConstString and also without a boolean to indicate if the string is mangled where we will now auto-detect if the string is mangled.

llvm-svn: 160450