Updated the XFAIL comment in variant tests.

Summary:
`ConstexprTestTypes::NoCtors` is an aggregate type (and consequently a literal type) in C++17,
but not in C++14 since it has a base class. This patch updates the comment to accurately describe the reason for the XFAIL.

Reviewers: EricWF

Reviewed By: EricWF

Differential Revision: https://reviews.llvm.org/D30481

llvm-svn: 296558