Fix bug in computing POD-for-layout.

A class with a field of non-POD-for-layout type is not POD-for-layout.
This computation should not depend on whether the field is of POD type
in the language sense.

Fixes PR16537.

Patch by Josh Magee.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186741 91177308-0d34-0410-b5e6-96231b3b80d8