[test] Cleanup nullopt_t tests

* Update specification text from N4387

* Delete not_brace_initializable.fail.cpp: it's redundant with nullopt_t.fail.cpp

* is_empty<T> implies is_class<T>

* is_literal is deprecated; directly verify that we can create a nullopt_t in a constexpr context

Differential Revision: D37024

llvm-svn: 312256