Define String8 and String16 operator<=>

Previously, in C++20 mode, using <=> on String8 or String16 would
compare the pointer values from the implicit conversion operators
returning const char* and const char16_t*. Instead, compare the string
content. This is especially a problem with STL containers that prefer
<=> when it is available.

See https://godbolt.org/z/cc1hW17h3 for a demo of the problem.

Bug: 339775405
Test: treehugger
Change-Id: I5ee6af96dee0c9968a1bab1ad96729e45bb1ac50