Fix memory leak in SubsPrimitiveParmItanium

[teemperor]

Summary

FastDemangle gives us a C-string that we own (which is allocated in SymbolDemangler::GetDemangledCopy).
As we are not deleting the string, we leak memory whenever we call SubsPrimitiveParmItanium.

source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp

if (s.startswith(search)) {
       ptrdiff_t replace_idx = (mangled.size() - s.size()) + replaced_offset;
       output_buf.erase(replace_idx, search.size());
       output_buf.insert(replace_idx, replace.str());
     }
   };
 
   // FastDemangle will call our hook for each instance of a primitive type,
   // allowing us to perform substitution
-  const char *const demangled =
+  char *const demangled =
       FastDemangle(mangled.str().c_str(), mangled.size(), swap_parms_hook);
 
   if (log)
     log->Printf("substituted mangling for %s:{%s} %s:{%s}\n",
                 mangled.str().c_str(), demangled, output_buf.c_str(),
                 FastDemangle(output_buf.c_str()));
+  // FastDemangle malloc'd this string.
+  free(demangled);
 
   return output_buf == mangled ? ConstString() : ConstString(output_buf);
 }
 
 uint32_t CPlusPlusLanguage::FindAlternateFunctionManglings(
     const ConstString mangled_name, std::set<ConstString> &alternates) {
   const auto start_size = alternates.size();
   /// Get a basic set of alternative manglings for the given symbol `name`, by