[ADT] Reimplement operator==(StringRef, StringRef) (NFC) (#91139)

I'm planning to deprecate and eventually remove StringRef::equals in
favor of operator==.  This patch reimplements operator== without using
StringRef::equals.

I'm not sure if there is a good way to make StringRef::compareMemory
available to operator==, which is not a member function.  "friend"
works to some extent but breaks corner cases, which is why I've chosen
to "inline" compareMemory.