[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/Plugins/ObjectFile/PECOFF/ObjectFilePECOFF.cpp

@@ -1092,7 +1092,7 @@ llvm::Optional<FileSpec> ObjectFilePECOFF::GetDebugLink() {
   uint32_t gnu_debuglink_crc;
   if (GetDebugLinkContents(*m_binary, gnu_debuglink_file, gnu_debuglink_crc))
     return FileSpec(gnu_debuglink_file);
-  return llvm::None;
+  return std::nullopt;
 }
 
 uint32_t ObjectFilePECOFF::ParseDependentModules() {