[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/Process/Linux/NativeProcessLinux.cpp

@@ -1877,13 +1877,13 @@ static llvm::Optional<WaitStatus> HandlePid(::pid_t pid) {
       -1, ::waitpid, pid, &status, __WALL | __WNOTHREAD | WNOHANG);
 
   if (wait_pid == 0)
-    return llvm::None;
+    return std::nullopt;
 
   if (wait_pid == -1) {
     Status error(errno, eErrorTypePOSIX);
     LLDB_LOG(log, "waitpid({0}, &status, _) failed: {1}", pid,
              error);
-    return llvm::None;
+    return std::nullopt;
   }
 
   assert(wait_pid == pid);