[libcxx] Marks transform_error.mandates.verify.cpp unsupported on msvc

PR llvm#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 marks this test as unsupportted
on msvc so both bots using ToT clang and release version clang can be
green.

Author: zeroomega

libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp

@@ -10,11 +10,12 @@
 // minimum required version these obsolete tests can be removed.
 // TODO(LLVM-20) remove spurious clang diagnostic tests.
 
-// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-
 // With clang-cl, some warnings have a 'which is a Microsoft extension' suffix
-// which break the tests.
-// XFAIL: msvc
+// which break the tests. But #102851 will turn it into an error, making the test pass.
+// However, upstream libcxx buildbots do not build clang from source while testing, so
+// this tests still expected to fail on these bots. Mark msvc as unsupported to mitigate
+// this issue.
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, msvc
 
 // Test the mandates
 

libcxx/test/libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp

@@ -10,11 +10,12 @@
 // minumum required version these obsolete tests can be removed.
 // TODO(LLVM-20) remove spurious clang diagnostic tests.
 
-// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-
 // With clang-cl, some warnings have a 'which is a Microsoft extension' suffix
-// which break the tests.
-// XFAIL: msvc
+// which break the tests. But #102851 will turn it into an error, making the test pass.
+// However, upstream libcxx buildbots do not build clang from source while testing, so
+// this tests still expected to fail on these bots. Mark msvc as unsupported to mitigate
+// this issue.
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, msvc
 
 // Test the mandates