Clang: Forward compiler options/flags to libclang

Currently only the defines and include paths options are forwarded to libclang. Forwarding the other options from the project manager is important, too:

• Warning options might help the user before invoking the build step.
• Certain flags might set certain defines, which could be checked by headers in an unfavorable way, consider:

...
#if !defined(QT_BOOTSTRAPPED) && defined(QT_REDUCE_RELOCATIONS) && defined(__ELF__) && \
    (!defined(__PIC__) || (defined(__PIE__) && defined(Q_CC_GNU) && Q_CC_GNU >= 500))
#  error "You must build your code with position independent code if Qt was built with -reduce-relocations. "\
         "Compile your code with -fPIC (-fPIE is not enough)."
#endif
...

The #error is problematic here.

Check if we can switch to the "clang-cl" driver for libclang. Then we could even forward the options for msvc.