Clang-Tidy: copy stdout to sterr; ignore original stderr

Clang-Tidy writes the number of warnings, the number of suppressed
warnings, and instructions on how to suppress warnings to stderr.  Since
each source file is checked individually, this repetitive information is
disturbing and should be suppressed.

The actual warning messages are written to stdout.  Some IDEs (eg.
QtCreator) analyze only stderr for issues.  Redirecting Clang-Tidy's
stdout to stderr makes sure the warnings are correctly displayed.