[libcxx] Start defining lit features for tests depending on availability

This patch removes some vendor-specific availability XFAILs from the
test suite. In the future, when a new feature is introduced in the
dylib, an availability macro should be created and a matching lit
feature should be created. That way, the test suite can XFAIL whenever
the implementation lacks the necessary feature instead of being
cluttered by vendor-specific annotations.

Right now, those vendor-specific annotations are still somewhat cluttering
the test suite by being in `config.py`, but at least they are localized.
In the future, we could design a way to define those less intrusively or
even automatically based on the availability macros that already exist
in <__config>.

llvm-svn: 353201