[clang][Darwin] Refactor header search path logic into the driver
This commit moves the logic for determining system, resource and C++
header search paths from CC1 to the driver. This refactor has already
been made for several platforms, but Darwin had been left behind.
This refactor tries to implement the previous search path logic with
perfect accuracy. In particular, the order of all include paths inside
CC1 and all paths that were skipped because nonexistent are conserved
after the refactor. This change was also tested against a code base
of significant size and revealed no problems.
Reviewers: jfb, arphaman
Subscribers: nemanjai, javed.absar, kbarton, christof, jkorous, dexonsmith, jsji, cfe-commits
Differential Revision: https://reviews.llvm.org/D61963