isPodLike: more precise

I tried to use isPodLike in:
  http://reviews.llvm.org/D18483

That failed because !is_class is too strict on platforms which don't yet
have is_trivially_copyable. This update tries to make isPodLike smarter
for platforms which don't have is_trivially_copyable, and AFAICT it
Should Just Work on all of them. I'll revert if the bots disagree with
me.

I'll also rename isPodLike to isTriviallyCopyable if this all works out,
since that's what the standard calls it now and one day we'll be rid of
isPodLike.

llvm-svn: 264541