Remove HostInfo::useDriverDriver(). This was only used in two places

inside the innards of the Driver implementation, and only ever
implemented to return 'true' for the Darwin OSes. Instead use a more
direct query on the target triple and a comment to document why the
target matters here.

If anyone is worried about this predicate getting wider use or improper
use, I can make it a local or private predicate in the driver.

llvm-svn: 148797