Stop diagnosing member and array access in offsetof as an extension

This was a mistake from e7300e75
(https://reviews.llvm.org/D133574) caused by us accidentally tracking
an older copy of the C DR list for DR496. The text in
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_496 makes
it clear that subobjects are allowed, which means member and array
access expressions are allowed.

This backs out the changes from the previous commit that relate to this
diagnostic.