Fix horrible regression in control of warnings caused by introduction of -Wpedantic.

While -Wpedantic was reasonable, -Wno-pedantic would turn off a bunch of warnings that
are on by default.  This counters the intention of this warning flag.

To fix this, -Wpedantic now includes extentions that are not on by default.  The
remaining warnings will manifest anyway, and won't accidentally get turned off
by -Wno-pedantic.

Fixes <rdar://problem/12076105>

llvm-svn: 161695