Since commit f6f4eb09 (clang-tidy: Add driver mode argument,
2020-01-19) the `clang-tidy` tool rejects the new option:

    Unknown command line argument '--driver-mode=g++'.

This is because we are adding the flag before the main compiler
command-line.  Encode it with `--extra-arg-before=` so that `clang-tidy`
knows it is supposed to be part of the compiler command.

Suggested-by: Hanjiang Yu