Add back --sysroot support for darwin header search.

Before e97b5f5c ([clang][Darwin] Refactor header search path logic
into the driver), both --sysroot and -isysroot worked to specify where
to look for system and C++ headers on Darwin. However, that change
caused clang to start ignoring --sysroot.

This fixes the regression, and adds tests.

(I also note that on all other platforms, clang seems to almost
completely ignore -isysroot, but that's another issue...)

Differential Revision: https://reviews.llvm.org/D62268

llvm-svn: 361429