[lit] Add an option to print all features used in tests

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 Lit option '--show-used-features' that prints all
the features used in XFAIL, REQUIRES and UNSUPPORTED of all tests that
are discovered.

Differential Revision: https://reviews.llvm.org/D78589