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.
Also, clean up InitHeaderSearch, making it clearer which targets manage header search paths in the driver.
Add test.
I think Fuchsia way of checking if (!D.SysRoot.empty()) { before adding /usr/include is probably better. Is it /include or /usr/include ?