[lldb][NFC] Use AppendEmptyArgument in CompletionRequest constructor
[lldb][NFC] Use AppendEmptyArgument in CompletionRequest constructor

We now have a utility function for this purpose.

(Also fixing the typo in the related comment while I'm at it.)

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@372946 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb