clang-tidy: Fix explicit --driver-mode= argument
Since !4208 (merged) 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: @de1acr0ix