This commit enforces on the whole project the usage of our own assertion
system. This means all calls to `abort`, `assert` and `llvm_unreachable`
have been replaced with calls to `revng_abort`, `revng_assert` and
`revng_unreachable`, respectively.

The error messages, usually expressed as `assert(Condition &&
"Message")` have now been replaced using the second (optional) argument
of `revng_assert`.

Additionally, all the `assert(false)` statements have been replaced with
calls to `revng_abort`. Apart from readibility, this ensures the
compilers is aware of the fact that call will never return.

This change will enable us to drop many statements whose sole purpose
was marking a variable employed in an assertion as used in release
mode. In fact, with the new assertion system this is no longer
necessary.