[lldb] Use std::nullopt instead of None (NFC)

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

lldb/source/Interpreter/ScriptInterpreter.cpp

@@ -91,7 +91,7 @@ llvm::Optional<MemoryRegionInfo>
 ScriptInterpreter::GetOpaqueTypeFromSBMemoryRegionInfo(
     const lldb::SBMemoryRegionInfo &mem_region) const {
   if (!mem_region.m_opaque_up)
-    return llvm::None;
+    return std::nullopt;
   return *mem_region.m_opaque_up.get();
 }