Some time ago, I introduced shortcut features like dylib-has-no-shared_mutex
to encode whether the deployment target supported shared_mutex (say). This
made the test suite annotations cleaner.
However, the problem with building Lit features on top of other Lit
features is that it's easier for them to become stale, especially when
they are generated programmatically. Furthermore, it makes the bar for
defining configurations from scratch higher, since more features have
to be defined. Instead, I think it's better to put the XFAILs in the
tests directly, which allows cleaning them up with a simple grep.
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