[Driver] move FreeBSD header search path management to the driver
This matches OpenBSD, and it supports Swift's use of clang for its C interop
functionality. Recent changes to Swift use AddClangSystemIncludeArgs() to
inspect the cc1 args; this doesn't work for platforms where cc1 adds standard
include paths implicitly. See:
<https://github.com/apple/swift/commit/
cf3354222d9f480de74db390f53a6dcc749fde14>
Also clean up InitHeaderSearch, making it clearer which targets manage header
search paths in the driver.
Differential Revision: https://reviews.llvm.org/D138183