[lldb] Prevent GetDisplayDemangledName from returning empty string (#8965)

(cherry-picked from commit c4654ad)

Author: kastiglione

lldb/include/lldb/Target/Language.h

@@ -283,7 +283,7 @@ class Language : public PluginInterface {
     return mangled.GetMangledName();
   }
 
-  virtual ConstString GetDisplayDemangledName(Mangled mangled) const {
+  virtual ConstString GetDisplayDemangledName(const Mangled &mangled) const {
     return mangled.GetDemangledName();
   }
 

lldb/source/Core/Mangled.cpp

@@ -364,7 +364,8 @@ ConstString Mangled::GetDisplayDemangledName(
 // END SWIFT
 
   if (Language *lang = Language::FindPlugin(GuessLanguage()))
-    return lang->GetDisplayDemangledName(*this);
+    if (ConstString display_name = lang->GetDisplayDemangledName(*this))
+      return display_name;
   return GetDemangledName();
 }