Fix GetDisplayName when only a demangled name is available

[fjricci]

Summary

GetDisplayDemangledName will already return a ConstString() when
there is neither a mangled name or a demangled name, so we don't need to special
case here. This will fix GetDisplayName in cases where m_mangled contains
only a demangled name and not a mangled name.

source/Symbol/Function.cpp

bool Function::IsTopLevelFunction() {
 
   if (Language *language = Language::FindPlugin(GetLanguage()))
     result = language->IsTopLevelFunction(*this);
 
   return result;
 }
 
 ConstString Function::GetDisplayName() const {
-  if (!m_mangled)
-    return ConstString();
   return m_mangled.GetDisplayDemangledName(GetLanguage());
 }
 
 CompilerDeclContext Function::GetDeclContext() {
   ModuleSP module_sp = CalculateSymbolContextModule();
 
   if (module_sp) {
     SymbolVendor *sym_vendor = module_sp->GetSymbolVendor();

source/Symbol/Symbol.cpp

void Symbol::Clear() {
   m_addr_range.Clear();
 }
 
 bool Symbol::ValueIsAddress() const {
   return m_addr_range.GetBaseAddress().GetSection().get() != nullptr;
 }
 
 ConstString Symbol::GetDisplayName() const {
-  if (!m_mangled)
-    return ConstString();
   return m_mangled.GetDisplayDemangledName(GetLanguage());
 }
 
 ConstString Symbol::GetReExportedSymbolName() const {
   if (m_type == eSymbolTypeReExported) {
     // For eSymbolTypeReExported, the "const char *" from a ConstString
     // is used as the offset in the address range base address. We can
     // then make this back into a string that is the re-exported name.