[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/Breakpoint/BreakpointResolverFileLine.cpp

@@ -210,7 +210,7 @@ void BreakpointResolverFileLine::DeduceSourceMapping(
         return a;
       }
     }
-    return llvm::None;
+    return std::nullopt;
   };
 
   FileSpec request_file = m_location_spec.GetFileSpec();