[libc++][NFC] Rename the template parameter of __make_transparent (#186435)

Renaming from _Tp to _ArgumentType makes it clearer that we're passing
the argument type intended for the comparator, which allows checking
whether *that specific use* of the comparator would be transparent.

Fixes #186396

GitOrigin-RevId: 3ae428ff3a526247aff323445471fac55c15fc8c