Mangled: Fix an 'unused variable' warning on GNU/Linux

[majnemer]

Summary

Platforms which don't use LLDB's built-in demangler don't use the
'mangled_length' variable. Instead, replace it's only use by an
expression it is equivalent to.

[majnemer]

Closed by commit rL213681 (authored by @majnemer).

source/Core/Mangled.cpp

Mangled::GetDemangledName () const
     {
         // We need to generate and cache the demangled name.
         Timer scoped_timer (__PRETTY_FUNCTION__,
                             "Mangled::GetDemangledName (m_mangled = %s)",
                             m_mangled.GetCString());
 
         // Don't bother running anything that isn't mangled
         const char *mangled_cstr = m_mangled.GetCString();
-        long mangled_length = m_mangled.GetLength();
         if (cstring_is_mangled(mangled_cstr))
         {
             if (!m_mangled.GetMangledCounterpart(m_demangled))
             {
                 // We didn't already mangle this name, demangle it and if all goes well
                 // add it to our map.
 #ifdef LLDB_USE_BUILTIN_DEMANGLER
                 // Try to use the fast-path demangler first for the
                 // performance win, falling back to the full demangler only
                 // when necessary
                 char *demangled_name = FastDemangle (mangled_cstr,
-                                                     mangled_length);
+                                                     m_mangled.GetLength());
                 if (!demangled_name)
                     demangled_name = __cxa_demangle (mangled_cstr, NULL, NULL, NULL);
 #elif defined(_MSC_VER)
                 // Cannot demangle on msvc.
                 char *demangled_name = nullptr;
 #else
                 char *demangled_name = abi::__cxa_demangle (mangled_cstr, NULL, NULL, NULL);
 #endif