[libcxx][ranges] removes default_initializable from weakly_incrementable and view

also:

* removes default constructors from predefined iterators
* makes span and string_view views

Partially implements P2325.
Partially resolves LWG3326.

Differential Revision: https://reviews.llvm.org/D102468

GitOrigin-RevId: 462f8f06113616ac5646144972d3f453639aac69
19 files changed