Cleanup some test issues:

* improve coverage in `span`'s "conversion from `std::array`" test, while eliminating MSVC diagnostics about `testConstructorArray<T>() && testConstructorArray<const T, T>()` being redundant when `T` is already `const`.

* Remove use of `is_assignable` that triggers UB due to an insufficiently-complete type argument in `std::function`'s assignment operator test.

* Don't test that `shared_ptr` initialization from an rvalue triggers the lvalue aliasing constructor on non-libc++; this is not the case for Standard Libraries that implement LWG-2996. (Ditto, I'd simply remove this but it's your library ;).)

Differential Revision: https://reviews.llvm.org/D80030