[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