[Driver] move DragonFly header search path management to the driver

As has been done for other OS's. Move the header path management to the driver.

Also I noticed with D89690 that the library paths were updated for GCC 8, but
the C++ header path was not. So I also fixed that while here.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D158376