We currently call a lot of functions with the same list of types. To avoid forgetting any of them, this patch adds type_lists and utilities for it. Specifically, it adds
- `type_list` - This is just a list of types
- `concatenate` - This allows concatenating type_lists
- `for_each` - Iterate over a type_list
Reviewed By: ldionne, #libc
Spies: jloser, EricWF, libcxx-commits
Differential Revision: https://reviews.llvm.org/D137476
1323461fe7