[libc++] [test] Change a lot of free begin/end pairs to members. NFCI.

[libc++] [test] Change a lot of free begin/end pairs to members. NFCI.

If you have a `begin() const` member, you don't need a `begin()` member
unless you want it to do something different (e.g. have a different return
type). So in general, //view// types don't need `begin()` non-const members.

Also, static_assert some things about the types in "types.h", so that we
don't accidentally break those properties under refactoring.

Differential Revision: https://reviews.llvm.org/D111231