Use a CompletionRequest in the expression command completion [NFC]

The patch was originally written before we had a CompletionRequest,
so it still used a StringList to pass back the completions to
the request.

llvm-svn: 341124
diff --git a/lldb/source/Commands/CommandObjectExpression.cpp b/lldb/source/Commands/CommandObjectExpression.cpp
index fdeaac2..b880cbfd 100644
--- a/lldb/source/Commands/CommandObjectExpression.cpp
+++ b/lldb/source/Commands/CommandObjectExpression.cpp
@@ -369,9 +369,7 @@
   if (error.Fail())
     return 0;
 
-  StringList matches;
-  expr->Complete(exe_ctx, matches, cursor_pos);
-  request.AddCompletions(matches);
+  expr->Complete(exe_ctx, request, cursor_pos);
   return request.GetNumberOfMatches();
 }