This reapplies the change that was rolled back, with the change that it removes the convertibility requirement to string_view, because that excluded some desirable explicit conversions (like absl::Cord). Note that this is now the same criterion previously used by testing::internal::StringLike.
Fixes: #4912
PiperOrigin-RevId: 959205696
Change-Id: I002cb1224f5a2701bdf8e68ea4d91776b9c38593