[libc++] Add UNSUPPORTED for clang-14 since the underlying bug hasn't been fixed yet

This started breaking in the CI because we bumped the Clang version to 14,
which requires adjusting the markup in the test suite. I think it's actually
nice the we need to do that and that it doesn't happen automatically, since
it serves as a reminder that this is broken in Clang.

Author: ldionne

libcxx/test/std/utilities/tuple/tuple.tuple/tuple.assign/convert_copy.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // Triggers a Clang assertion: https://bugs.llvm.org/show_bug.cgi?id=45879
-// UNSUPPORTED: clang-13
+// UNSUPPORTED: clang-13, clang-14
 
 // <tuple>