XFAILing tests on host features

Some tests, such as llvm/test/tools/llvm-cov/range_based_for.cpp are expected to fail on big-endian systems. This is a property of the host, but the tests use XFAIL with a list of triple substrings. The documentation indicates that triple substrings are evaluated with respect to the target triple (and not the host triple).

It seems these tests should instead use REQUIRES on a (new) host feature for “host-little-endian”.

– HT