[lldb] Refactor OptionValue to return a std::optional (NFC)

Refactor OptionValue to return a std::optional instead of taking a fail
value. This allows the caller to handle situations where there's no
value, instead of being unable to distinguish between the absence of a
value and the value happening the match the fail value. When a fail
value is required, std::optional::value_or() provides the same
functionality.

lldb/source/Plugins/ObjectFile/PECOFF/ObjectFilePECOFF.cpp

@@ -319,7 +319,8 @@ size_t ObjectFilePECOFF::GetModuleSpecifications(
   llvm::Triple::EnvironmentType env;
   if (module_env_option)
     env =
-        (llvm::Triple::EnvironmentType)module_env_option->GetEnumerationValue();
+        (llvm::Triple::EnvironmentType)module_env_option->GetEnumerationValue()
+            .value_or(0);
   else
     env = GetGlobalPluginProperties().ABI();