This type trait had no precise definition, and indeed not even any documentation
at all. Giving the ill-defined concept a name was harmful, because it obscured
several places where the concept was used too conservatively, or even flat-out
incorrectly.

This CL has no behavior change: it simply expands IsMemcpyOk to the same
conditions it previously had. It leaves TODOs for each place where this was too
conservative or incorrect.

PiperOrigin-RevId: 519278325
Change-Id: I25bc89f299f6e40b5c3bce7370ed90f33a95612f