Lit test suites can tend to accumulate annotations that are not necessarily
relevant as time goes by, for example XFAILS on old compilers or platforms.
To help spot old annotations that can be cleaned up, it can be useful to
look at all features used inside a test suite.
This commit adds a new option '--show-features' that prints all the
features used in XFAIL, REQUIRES and UNSUPPORTED of all tests that