Nonsensically, the default header is not included by default. Users
are using the cc1 flag to compile OpenCL programs, which should not be
required. Switch the default, and allow -nostdinc to disable the
I'm somewhat confused by the way this is supposed to work, or why a
separate option exists for OpenCL. The other language flags seem to be
implemented in the driver, not cc1 like OpenCL. I'm not sure this is
the right set of flag naming decisions, or if we really need the
fno-include-default-header cc1 flag.