We currently define the preferred names in multiple places. `basic_string` and `basic_string_view` also have a lot of aliases, which makes the declarations quite long. So let's only add the preferred names in forward-declaring headers to make the implementation more readable and have all the preferred names in one place.
Reviewed By: ldionne
Spies: EricWF, krytarowski, libcxx-commits
Differential Revision: https://reviews.llvm.org/D135824
