We've been improving these the tests for vector quite a bit and we are
probably not done improving our container tests. Formatting everything
at once will make subsequent reviews easier.
- add the `from_range_t` constructors and the related deduction guides;
- add the `insert_range`/`assign_range`/etc. member functions.
(Note: this patch is split from https://reviews.llvm.org/D142335)
Differential Revision: https://reviews.llvm.org/D149830