[lldb] Use std::optional instead of llvm::Optional (NFC)

This patch replaces (llvm::|)Optional< with std::optional<.  I'll post
a separate patch to clean up the "using" declarations, #include
"llvm/ADT/Optional.h", etc.

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

@@ -1088,7 +1088,7 @@ UUID ObjectFilePECOFF::GetUUID() {
   return m_uuid;
 }
 
-llvm::Optional<FileSpec> ObjectFilePECOFF::GetDebugLink() {
+std::optional<FileSpec> ObjectFilePECOFF::GetDebugLink() {
   std::string gnu_debuglink_file;
   uint32_t gnu_debuglink_crc;
   if (GetDebugLinkContents(*m_binary, gnu_debuglink_file, gnu_debuglink_crc))