Fix comment to more accurately describe C++ language requirements around tail padding.

Summary:
As of C++ core issue 43 (http://wg21.link/cwg43), which was voted into
the C++ working draft in 1999, it is not permissible to memcpy a base
class subobject, even if it's of POD type, so there is no problem with
reusing the tail padding of a base class. That issue was voted into the
standard in DR status, so it applies retroactively to C++98 (and is in
any case part of C++03).

So stop suggesting that AlwaysUseTailPadding mode is non-conforming.

Reviewers: rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70923