No longer pass a StringRef to the Python API

Summary:
The refactoring patch for DoExecute missed this case of a variadic function that just silently
accepts a StringRef which it then tries to reinterpret as a C-string.

This should fix the Windows builds.

Reviewers: stella.stamenova

Reviewed By: stella.stamenova

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D49309

llvm-svn: 337030