[lldb] Make TestExprDiagnostics.py pass again after enabling Fix-Its in test

Commit 83c81c0a enabled Fix-Its for top-level
expressions which change the error message of this test here as Clang comes
up with a strange Fix-It for this expression. This patch just changes the
test to declare a void variable so that Clang doesn't see a way to
recover with a Fix-It and change the error message.