Extend the "__is_pod" hack, which demotes various type trait keywords

(__is_pod, __is_signed, etc.) to normal identifiers if they are
encountered in certain places in the grammar where we know that prior
versions of libstdc++ or libc++ use them, to still allow the use of
these keywords as type traits. Fixes <rdar://problem/9836262> and PR10184.

llvm-svn: 162937