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

libcxx/test/std/utilities/variant/variant.variant/variant.status/index.pass.cpp

@@ -10,7 +10,10 @@
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
 
-// The following compilers don't allow constexpr variables of non-literal type.
+// The following compilers don't consider a type an aggregate type (and
+// consequently not a literal type) if it has a base class at all.
+// In C++17, an aggregate type is allowed to have a base class if it's not
+// virtual, private, nor protected (e.g. ConstexprTestTypes:::NoCtors).
 // XFAIL: gcc-5, gcc-6
 // XFAIL: clang-3.5, clang-3.6, clang-3.7, clang-3.8
 // XFAIL: apple-clang-6, apple-clang-7, apple-clang-8

libcxx/test/std/utilities/variant/variant.variant/variant.status/valueless_by_exception.pass.cpp

@@ -10,7 +10,10 @@
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
 
-// The following compilers don't allow constexpr variables of non-literal type.
+// The following compilers don't consider a type an aggregate type (and
+// consequently not a literal type) if it has a base class at all.
+// In C++17, an aggregate type is allowed to have a base class if it's not
+// virtual, private, nor protected (e.g. ConstexprTestTypes:::NoCtors).
 // XFAIL: gcc-5, gcc-6
 // XFAIL: clang-3.5, clang-3.6, clang-3.7, clang-3.8
 // XFAIL: apple-clang-6, apple-clang-7, apple-clang-8