Blender Git Loki

BLI: add macro to indicate unreachable code

This adds a `BLI_assert_unreachable()` macro, that should be used instead
of `BLI_assert(false)` in many places.

* `BLI_assert_unreachable` provides more information than `BLI_assert(false)`
to people reading the code.
* `BLI_assert_unreachable` will print an error to `stderr` in a release build.
This makes it more likely that we will get bug reports when the assumptions
of a developer were wrong.

Differential Revision: https://developer.blender.org/D10780