PR #102851 marks reference types in union as error on msvc by changing
the clang, which makes 'transform_error.mandates.verify.cpp' no longer
failing on msvc from ToT. However, all libcxx buildbots do not build
clang from source, therefore, this test will still fail on these bots,
which is incorrect. This patch changed the expected error message of
this test so it can pass with both release branch clang and ToT clang.